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0 ta Underwater sound equipment. 

ta Summary technical report of the National Defense Research 
Committee 

ti Spine title on volume 1: ta Sonar listening systems 
ti Spine title on volume 2: ta Sonar echo-ranging systems 
ti Spine title on volume 3: ta Scanning sonar systems 
ti Spine title on volume 4: ta FM sonar systems 
ti Spine title on volume 5: ta Sonar instruments 
ti Spine title on volume 6: ta Sonar countermeasures 

1 ta Washington, D.C. : tb Office of Scientific Research and 
Development, National Defense Research Committee, Division 6, tc 
1946. 

ta 6 volumes : tb illustrations ; tc 28 cm. 
ta text tb txt %! rdacontent 
ta unmediated tb n t2 rdamedia 
ta volume tb nc t2 rdacarrier 

ta Summary technical report of Division 6, NDRC ; tv volume 14-19 


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Defense Research Committee. 




Bibliographic Record #18907115 


3/31/2016 9:56:32 AM 


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ta "Manuscript and illustrations for this volume were prepared for 
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bound by the Columbia University Press"--Unnumbered page ii. 

ta LC Science, Business & Technology copy no. 171 (volume 1), 239 
(volume 2), 3 (volume 3), 196 (volume 4), 64 (volume 5), 71 (volume 
6). t5 DLC 

Ta In a set of declassified documents held as a collection by the 

Library of Congress. T5 DLC 

ta Includes bibliographical references and indexes. 

ta I. Listening systems - II. Echo-ranging systems ~ III. Scanning 

sonar systems - IV. Frequency-modulated sonar systems - V. 

Instruments, attack aids and miscellaneous ordnance - VI. 

Countermeasures. 

0 ta Sonar. 

0 ta Underwater acoustics tx Instruments. 

0 ta Naval research tz United States. 

ta United States, tb Office of Scientific Research and Development, 
tb National Defense Research Committee, te issuing body. 



I 


SUMMARY TECHNICAL REPORT 
OF THE 

NATIONAL DEFENSE RESEARCH COMMITTEE 


This document contains information affecting the national defense of the 
United States within the meaning of the Espionage Act, 50 U. S. C., 31 
and 32, as amended. Its transmission or the revelation of its contents in 
any manner to an unauthorized person is prohibited by law. 

This volume is classified CONFIDENTIAL in accordance with security 
regulations of the War and Navy Departments because certain chapters 
contain material which was CONFIDENTIAL at the date of printing. 
Other chapters may have had a lower classification or none. The reader 
is advised to consult the War and Navy agencies listed on the reverse of 
this page for the current classification of any material. 


C 


I 


Manuscript and illustrations for this volume were prepared 
for publication by the Summary Reports Group of the Colum- 
bia University Division of War Research under contract 
OEMsr-1131 with the Office of Scientific Research and Devel- 
opment. This volume was printed and bound by the Columbia 
University Press. 

Distribution of the Summary Technical Report of NDRC has 
been made by the War and Navy Departments. Inquiries con- 
cerning the availability and distribution of the Summary 
Technical Report volumes and microfilmed and other reference 
material should be addressed to the War Department Library, 
Room lA-522, The Pentagon, Washington 25, D. C., or to the 
Office of Naval Research, Navy Department, Attention : Re- 
ports and Documents Section, Washington 25, D. C. 

Copy No. 

1^1 


This volume, like the seventy others of the Summary Techni- 
cal Report of NDRC, has been written, edited, and printed 
under great pressure. Inevitably there are errors which have 
slipped past Division readers and proofreaders. There may be 
errors of fact not known at time of printing. The author has 
not been able to follow through his writing to the final page 
proof. 

Please report errors to : 

JOINT RESEARCH AND DEVELOPMENT BOARD 
PROGRAMS DIVISION (STR ERRATA) 

WASHINGTON 25, D. C. 

A master errata sheet will be compiled from these reports and 
sent to recipients of the volume. Your help will make this book 
more useful to other readers and will be of great value in pre- 
paring any revisions. 




AL 


SUMMARY TECHNICAL REPORT OF DIVISION 6, NDRC 


VOLUME 14 


Underwater Sound Equipment I 

LISTENING SYSTEMS 


OFFICE OF SCIENTIFIC RESEARCH AND DEVELOPMENT 
VANNEVAR BUSH, DIRECTOR 

NATIONAL DEFENSE RESEARCH COMMITTEE 
JAMES B. CONANT, CHAIRMAN 

DIVISION 6 
JOHN T. TATE, CHIEF 


WASHINGTON, D. C., 1946 


CO Nj 'lDI iNTfe U. 


NATIONAL DEFENSE RESEARCH COMMITTEE 


James B. Conant, Chairman 
Richard C. Tolman, Vice Chairman 
Roger Adams Army Representative ^ 

Frank B. Jewett Navy Representative ^ 

Karl T. Compton Commissioner of Patents ^ 
Irvin Stewart, Executive Secretary 


Army representatives in order of service: 


^Navy representatives in order of service: 


Maj. Gen. G. V. Strong 
Maj. Gen. R. C. Moore 
Maj. Gen. C. C. Williams 
Brig. Gen, W. A. Wood, Jr. 

Col. E. A. 


Col. L. A. Denson 
Col. P. R. Faymonville 
Brig. Gen. E. A. Regnier 
Col. M. M. Irvine 
Routheau 


Rear. Adm. H. G. Bowen Rear Adm. J. A. Purer 

Capt. Lybrand P. Smith Rear Adm. A. H. Van Keuren 

Commodore H. A. Schade 
3 Commissioners of Patents in order of service: 
Conway P. Coe Casper W. Corns 


NOTES ON THE ORGANIZATION OF NDRC 


The duties of the National Defense Research Committee 
were (1) to recommend to the Director of OSRD suitable 
projects and research programs on the instrumentalities 
of warfare, together with contract facilities for carrying 
out these projects and programs, and (2) to administer 
the technical and scientific work of the contracts. More 
specifically, NDRC functioned by initiating research 
projects on requests from the Army or the Navy, or on 
requests from an allied government transmitted through 
the Liaison Office of OSRD, or on its own considered ini- 
tiative as a result of the experience of its members. Pro- 
posals prepared by the Division, Panel, or Committee for 
research contracts for performance of the work involved 
in such projects were first reviewed by NDRC, and if 
approved, recommended to the Director of OSRD. Upon 
approval of a proposal by the Director, a contract permit- 
ting maximum flexibility of scientific effort was arranged. 
The business aspects of the contract, including such mat- 
ters as materials, clearances, vouchers, patents, priori- 
ties, legal matters, and administration of patent matters 
were handled by the Executive Secretary of OSRD. 

Originally NDRC administered its work through five 
divisions, each headed by one of the NDRC members. 
These were: 


Division 

Division 

Division 

Division 

Division 


A — Armor and Ordnance 
B — Bombs, Fuels, Gases, & Chemical Prob- 
lems 

C — Communication and Transportation 
D — Detection, Controls, and Instruments 
E — Patents and Inventions 


In a reorganization in the fall of 1942, twenty-three ad- 
ministrative divisions, panels, or committees were cre- 
ated, each with a chief selected on the basis of his out- 
standing work in the particular field. The NDRC mem- 
bers then became a reviewing and advisory group to the 
Director of OSRD. The final organization was as follows: 


Division 1 — Ballistic Research 

Division 2 — Effects of Impact and Explosion 

Division 3 — Rocket Ordnance 

Division 4 — Ordnance Accessories 

Division 5 — New Missiles 

Division 6 — Sub-Surface Warfare 

Division 7 — Fire Control 

Division 8 — Explosives 

Division 9 — Chemistry 

Division 10 — Absorbents and Aerosols 

Division 11 — Chemical Engineering 

Division 12 — Transportation 

Division 13 — Electrical Communication 

Division 14 — Radar 

Division 15 — Radio Coordination 

Division 16 — Optics and Camouflage 

Division 17 — Physics 

Division 18 — War Metallurgy 

Division 19 — Miscellaneous 

Applied Mathematics Panel 

Applied Psychology Panel 

Committee on Propagation 

Tropical Deterioration Administrative Committee 


Library of Congress 

lC(^»MftTIAL 

2015 490958 



NDRC FOREWORD 


A s EVENTS of the years preceding 1940 re- 
^vealed more and more clearly the serious- 
ness of the world situation, many scientists 
in this country came to realize the need of or- 
ganizing scientific research for service in a 
national emergency. Recommendations which 
they made to the White House were given 
careful and sympathetic attention, and as a 
result the National Defense Research Com- 
mittee [NDRC] was formed by Executive 
Order of the President in the summer of 1940. 
The members of NDRC, appointed by the Presi- 
dent, were instructed to supplement the work 
of the Army and the Navy in the development 
of the instrumentalities of war. A year later, 
upon the establishment of the Office of Scien- 
tific Research and Development [OSRD], 
NDRC became one of its units. 

The Summary Technical Report of NDRC is 
a conscientious effort on the part of NDRC to 
summarize and evaluate its work and to pre- 
sent it in a useful and permanent form. It 
comprises some seventy volumes broken into 
groups corresponding to the NDRC Divisions, 
Panels, and Committees. 

The Summary Technical Report of each Di- 
vision, Panel, or Committee is an integral sur- 
vey of the work of that group. The first volume 
of each group’s report contains a summary of 
the report, stating the problems presented and 
the philosophy of attacking them and summariz- 
ing the results of the research, development 
and training activities undertaken. Some vol- 
umes may be “state of the art” treatises cover- 
ing subjects to which various research groups 
have contributed information. Others may 
contain descriptions of devices developed in 
the laboratories. A master index of all these 
divisional, panel, and committee reports which 
together constitute the Summary Technical 
Report of NDRC is contained in a separate 
volume, which also includes the index of a 
microfilm record of pertinent technical labo- 
ratory reports and reference material. 

Some of the NDRC-sponsored researches 
which had been declassified by the end of 1945 
were of sufficient popular interest that it was 
found desirable to report them in the form of 
monographs, such as the series on radar by 
Division 14 and the monograph on sampling 
inspection by the Applied Mathematics Panel. 


Since the material treated in them is not dupli- 
cated in the Summary Technical Report of 
NDRC, the monographs are an important part 
of the story of these aspects of NDRC research. 

In contrast to the information on radar, 
which is of widespread interest and much of 
which is released to the public, the research 
on subsurface warfare is largely classified and 
is of general interest to a more restricted 
group. As a consequence, the report of Division 
6 is found almost entirely in its Summary 
Technical Report, which runs to over twenty 
volumes. The extent of the work of a Division 
cannot therefore be judged solely by the 
number of volumes devoted to it in the Sum- 
mary Technical Report of NDRC: account 
must be taken of the monographs and available 
reports published elsewhere. 

Any great cooperative endeavor must stand 
or fall with the will and integrity of the men 
engaged in it. This fact held true for NDRC 
from its inception, and for Division 6 under 
the leadership of Dr. John T. Tate. To Dr. Tate 
and the men who worked with him— some as 
members of Division 6, some as representatives 
of the Division’s contractors — belongs the 
sincere gratitude of the Nation for a difficult 
and often dangerous job well done. Their 
efforts contributed significantly to the outcome 
of our naval operations during the war and 
richly deserved the warm response they re- 
ceived from the Navy. In addition, their con- 
tributions to the knowledge of the ocean and 
to the art of oceanographic research will 
assuredly speed peacetime investigations in this 
field and bring rich benefits to all mankind. 

The Summary Technical Report of Division 
6, prepared under the direction of the Division 
Chief and authorized by him for publication, 
not only presents the methods and results of 
widely varied research and development pro- 
grams but is essentially a record of the un- 
stinted loyal cooperation of able men linked in 
a common effort to contribute to the defense 
of their Nation. To them all we extend our 
deep appreciation. 

Vannevar Bush, Director 
Office of Scientific Research and Development 

J. B. CONANT, Chairman 

National Defense Research Committee 


CO NF I DENTIAL 



FOREWORD 


I N SUBMARINE WARFARE the term listening is 
used in a generalized sense to describe a 
variety of ways in which a ship, submarine, or 
torpedo may be made to betray its presence and 
location to a listener by the noise it makes. 
Such noises are picked up by a hydrophone or 
an array of hydrophones and, after suitable 
combination and amplification, are made to 
operate a loudspeaker, recorder, or other indi- 
cating device. 

To be recognized by the ear, sound must be 
above a certain minimum intensity or energy 
level. Also, only those components of a sound 
are recognizable which lie in the frequency 
spectrum above a certain lower frequency and 
below a certain upper frequency. Further, to be 
recognized by the ear, a particular sound must 
not be masked by other sounds. This does not 
mean that sounds which, because of their low 
intensity are inaudible to the unaided ear, can- 
not by instrumental aids be made audible by 
amplification, nor that sounds outside of the 
audible frequency range cannot by instrumental 
methods be transformed to the audible range 
and thus made recognizable. Both industry and 
the military are vitally concerned with the art 
that has developed from these principles. 

In addition to the pure-listening art, it is 
frequently desired to utilize the energy arriving 
from a sound source for purposes other than 
audible recognition. For example, sound energy 
can be used to supplement auditory recognition 
by visual or mechanical instrumentation, or 
even to explode a mine, or guide a torpedo to 
the sound source. Many of the physical factors 
pertinent to listening likewise apply to these 
other applications. 

While listening techniques have many mili- 
tary applications, the program of Division 6 
was generally limited to their application in 
antisubmarine and prosubmarine ‘ warfare. 
Even so restricted, these techniques are criti- 
cally important. The NDRC and Navy agencies 
have made substantial progress in the develop- 
ment of listening methods, and large additions 
have been made to the knowledge of physical 
and physiological factors involved. However 
there is still room for much further research 
and development. Because sound is the only 
form of energy which water transmits without 
enormous energy losses as the distance from 
the source is increased, it appears that listen- 
ing techniques will continue to be important in 
subsurface warfare. 


The present volume, prepared by J. S. Cole- 
man, J. W. Horton, and D. A. Proudfoot, does 
not cover all matters pertinent to the listening 
art. It is primarily a description of a number 
of listening devices developed by Division 6 
with emphasis on instrumentation problems. 
Elsewhere in this series of technical reports 
are found the results of basic studies by the 
Division on the noise produced by surface ships, 
submarines, and torpedoes ; on the propagation, 
attenuation, and scattering of sound in the sea ; 
on background noise; on the effect of bottom 
and surface; and on countermeasures to enemy 
listening devices. 

The development of listening devices was for 
the rnost part assigned by the Division to Co- 
lumbia University’s New London Laboratory 
and to the Bell Telephone Laboratories. The 
Underwater Sound Reference Laboratories also 
contributed to the work of these laboratories 
by making available facilities for standardiz- 
ing measurements. Two factors are of supreme 
importance in the development of listening 
methods and devices : the behavior of sound in 
the ocean, and the performance of the human 
ear. As research continued at Woods Hole and 
San Diego, data on underwater sound were 
gradually accumulated and forwarded to the 
development engineers. The Bell Telephone 
Laboratories contributed to the project the 
results of many years of physiological research 
on hearing. 

This general activity proceeded under several 
Navy projects, always receiving the most help- 
ful support and liaison from the interested 
Bureaus. The Navy also furnished facilities 
for tests to supplement the facilities of the 
laboratories. In addition to maintaining close 
liaison with the Bureaus, in the later stages of 
development and operational use of devices, 
many contracts were made with COMINCH 
and the operating forces. In every case these 
contacts were most satisfactory. 

No attempt has been made to ascribe credit 
for technical performance either to those who 
directed the work in the various organizations 
or to the individuals on their staffs. However it 
is but due his memory to mention particularly 
Dr. Albert L. Thuras who, for over four years, 
was a member of the New London Laboratory. 
He brought to his task rare enthusiasm and 
experimental skill. His contribution to the 
various projects was outstanding. 

John T. Tate 
Chief, Division 6 


vii 








PREFACE 


T his report is intended to summarize the 
work of the Division 6 laboratories on un- 
derwater listening devices and techniques ap- 
plicable to the detection, navigation, and opera- 
tion of submarine craft. It was prepared by the 
Summary Reports Group of the Columbia Uni- 
versity Division of War Research on the basis 
of numerous technical progress and completion 
reports submitted by the participating con- 
tractors, together with certain supplementary 
material collected and organized into its present 
form by members and contributors to the group. 

The programs and developments recounted 
were, for the most part, undertaken by the 
Columbia University Underwater Sound Lab- 
oratory at New London, Connecticut, and the 
Bell Telephone Laboratories in New York. Both 
groups have contributed significantly to our 
total of knowledge in this field. It is regretted 
that the very large number of their staff mem- 
bers, whose accomplishments are only partially 
recorded here, makes it impossible to give in- 
dividual credit. 


The volume is not intended to serve in any 
sense as a textbook but as a reference useful 
to newcomers to this rather specialized art, who 
may wish to acquire background, as well as to 
those who may be interested in particular de- 
tails and the reasoning behind the electronic 
techniques and circuitry employed. 

As editor of the volume, I should like to ac- 
knowledge the assistance of Donald E. Proud- 
foot and William J. Meringer of the SRG staff, 
of J. Warren Horton of the Massachusetts In- 
stitute of Technology who prepared the chapter 
on Evolution of Listening Gear, and to the Bell 
Telephone Laboratories and the naval staff of 
the United States Navy Underwater Sound 
Laboratory at New London who were extremely 
helpful in providing the many charts, diagrams, 
and photographs which illustrate the text. 


John S. Coleman 
Editor 


CQ gFIiniD^llAr 


IX 




CONTEJNTS 


CHAPTER 

1 Introduction to Listening 

2 Evolution of Listening Gear 

3 Physical Factors Affecting the Target Signal .... 

4 Experimental Listening Systems 

5 Comparison of Experimental Systems 

6 Bearing Indicator Systems 

7 Surface Craft Listening Equipment — JP Systems . 

8 Aircraft Listening Equipment — Towed Hydrophones . 

9 Aircraft Listening Equipment — Radio Sono Buoys . . 

10 Submarine Listening Equipment — JP and JT Systems . 

11 The 692 Sonar System 

12 Submarine Triangulation-Listening-Ranging [TLR] System 

13 Torpedo Detection Studies and Systems — MVP and TDM 

14 Harbor Protection Systems 

15 Submarine Communication Systems 

Glossary 

Bibliography 

Contract Numbers 

Service Project Numbers 

Index 


PAGE 

1 

7 

17 

23 

43 

50 

61 

67 

73 

104 

122 

140 

154 

169 

182 

195 

197 

203 

204 

205 



XI 




Chapter 1 


INTRODUCTION TO LISTENING 


S OUND, BROADLY DEFINED, is the Only form of 
energy which can be transmitted any con- 
siderable distance underwater. Important civil- 
ian and military developments stem from this 
physical fact. In some cases a source of sound 
energy is intentionally provided, as in under- 
water communication and echo-ranging sys- 
tems, while in other cases the source of sound 
energy utilized is incidental to the operation 
of vessels or weapons in the water, such as 
surface ships, submarines, and torpedoes. Such 
sound energy may be employed to determine 
the presence and location of its source. In 
this process of determination, commonly called 
detection, the ability of the ear to discriminate 
and to identify characteristic sound patterns 
has stimulated development of a listening art 
and numerous pieces of gear termed listening 
devices. 

Although the energy radiated from a sound 
source such as a moving surface ship or sub- 
marine has had important uses for the Navy 
and the Division other than to determine the 
presence and location of the source, this report 
is limited to developments in the field of listen- 
ing gear and its supplementary devices. It 
does not include listening as a sequence of echo 
ranging, in which the reflecting surface of a 
structure in the water becomes, as it were, a 
sound source to be detected and located. Ad- 
vances in such applications of the listening art, 
however, naturally parallel those in “pure’^ 
listening, and it should be noted that echo- 
ranging gear may be used for listening to 
sounds other than the echo, a supplementary 
use which is of great tactical importance. The 
two arts of echo ranging and listening in fact 
very much overlap, and in actual military op- 
erations one supplements the other. 

In World War I, the problem of detecting 
and locating the submarine was attacked by 
modifying existing air listening devices to 
serve underwater both for submarine and sur- 
face craft. Since knowledge of the techniques 
of electronic amplification was scant and hy- 


drophones inefficient, such equipment was only 
moderately effective. That it was effective at 
all was attributable only to the extremely noisy 
submarines then in service. Following the tac- 
tical employment of listening equipment, a 
countereffort was made to quiet the submarine, 
particularly during the period of evasion. The 
success of this effort so reduced the range of 
listening gear as to make detection almost im- 
possible except at extremely short ranges. A 
partial answer was found in echo-ranging gear 
which, relying on the echo of its own signal, 
again permitted the attainment of useful target 
ranges. As no entirely successful counter- 
measure to reduce echo strength has yet been 
devised, this method has received the major 
development effort in the field of antisubmarine 
warfare. 

The same knowledge and developments, how- 
ever, that have advanced the design of echo- 
ranging gear have also advanced the design of 
listening gear, since listening is, essentially, 
the second half of echo ranging. Better hydro- 
phones, efficient amplifiers, and an increased 
understanding of the phenomena involved have 
brought corresponding extensions of range and 
flexibility of application. Although, in general, 
the extreme quietness of operation possible 
with modern submarines has made it inadvis- 
able to rely on listening gear for their positive 
detection, in certain tactical situations listen- 
ing offers a number of advantages that may 
have increasing importance. 

^ ^ VIRTUES OF LISTENING 

METHODS 

Tactical Security 

The operation of any detection system which 
depends upon echoes of its own signal can be 
detected in turn at far greater ranges than its 
own maximum range. The use of echo ranging 
or radar, therefore, not only may jeopardize 
own-ship’s security but also inherently presents 




I 


2 


INTRODUCTION TO LISTENING 


the possibility of giving the enemy more infor- 
mation than can be received. Worse, because of 
the range difference, it may warn the enemy 
out of range and thus destroy the chance of 
securing any information at all. It is some- 
times possible to select transmission frequen- 
cies or wavelengths which are believed not 
available to the enemy and thus obtain some 
measure of security. However, with the in- 
creasing use of panoramic receivers and the 
physical limitations of available spectrum, such 
a measure can only be regarded as temporary. 

The advantage of security offered by listen- 
ing is of particular importance to the sub- 
marine. This is true both when it is submerged 
and when it is surfaced, for, since no energy 
is radiated, it is possible for the submarine to 
secure information concerning the position and 
movements of the enemy without increasing 
its own detectability. In this respect, listen- 
ing is comparable to optical scanning methods 
when surfaced. With the submarine sub- 
merged, the security advantage of listening is 
greater, since the periscope is detectable by 
both radar and optical means. Also, the use 
of the periscope restricts the submarine to 
shallow submergence depths where it may be 
detected by its shadow or silhouette from the 
air. 

It is interesting to note that maximum range 
and bearing accuracy have been so improved in 
listening gear for submarines that a successful 
torpedo attack has been conducted using no 
other target information than that obtained 
from the listening gear. 

Maximum Range 

Except for very quiet targets, listening 
methods permit detection at a greater maxi- 
mum range than do echo ranging. The two 
methods provide equal ranges only when the 
signal-to-noise ratio of the returning echo is 
at least equal to that for the target noise signal. 
As the echo-ranging signal must travel the 
range distance twice and suffers also from im- 
perfect reflection by the target, it is subject 
to much greater losses than is the listening 
signal. Calculation of these losses shows that 
the original acoustic output necessary to secure 
a detectable echo from a typical target gets 


uncomfortably large for ranges exceeding a 
few thousand yards. Thus, while under favor- 
able conditions echo ranges of 2,000 to 4,000 
yards are considered good, submarines using 
listening gear often detect surface targets at 

20.000 yards and more. Unfortunately for 
surface craft applications of listening, modern 
submarines, by reducing speed and stopping 
auxiliaries, can be operated so quietly as to be 
completely inaudible at ranges greater than a 
few hundred feet. An equivalent tactic on the 
part of surface ships, however, would obviously 
result in the loss of their function if not in 
their destruction. 

In the course of World War II, American 
submarines were able to use radar for long- 
range search and were not forced to rely on 
listening gear. As long as no loss in ship secur- 
ity is involved, this method, giving both range 
and bearing, is to be recommended over pres- 
ent-day listening systems which give less ac- 
curate bearings and only approximate ranges. 
However, it is not unlikely that it will soon be 
necessary to maintain radar silence as well as 
radio silence. This probability, together with 
the advent of greatly increased speeds, and 
therefore noise, for both submarine and sur- 
face craft, indicates that listening will play a 
more important role in establishing long-range 
contact. 

Continuity of Information 

Listening methods provide continuous rather 
than intermittent information. While this fea- 
ture is less significant where high velocities of 
propagation permit extremely short cycling 
periods, as in the case of radar, it becomes a 
very real advantage in the case of sonar. Sound 
in water travels -at less than 1 mile per second. 
With conventional echo-ranging gear, this 
means that for maximum search ranges of 

4.000 yards information is received for only a 
few hundredths of a second every 5 seconds. 
At 1,000 yards this interval is reduced to about 
1.2 seconds. Further, because of the variability 
of amplitude, phase, and path taken by the echo 
and because of the difficulty in keeping the 
projector trained accurately on the target, it is 
generally necessary to take several readings. 


GONPII)lSjlAL 


FACTORS AFFECTING PERFORMANCE 


3 


The continuous flow of information received 
by listening gear, contrasted with echo rang- 
ing, not only permits adjusting gain and selec- 
tivity controls for optimum signal discrimina- 
tion but also, because of the operator’s ability 
to average with time, permits securing more 
accurate bearings. With the development of 
more precise bearing indicators, this ability of 
listening gear becomes of increasing interest. 
Precise bearings permit obtaining accurate 
bearing rates which are useful in computing 
target course and speed. Thus, with triangula- 
tion-listening-ranging [TLR] gear, it may soon 
be possible to supply all information necessary 
to a computer for a torpedo attack. 

Wide-Band Coverage 

Still another advantage of importance is the 
ability of listening gear to accept a wide- 
frequency spectrum. This enables simultaneous 
reception of the lower sonic frequencies which 
are useful in determining the nature of the 
target as well as the higher frequencies which 
permit accurate determination of bearing. Also 
it enables the operator to select quickly and 
concentrate upon any distinctive frequency 
band in which the target may be particularly 
noisy. This is possible because target sounds 
are rarely monochromatic except when the 
target is echo ranging, in which case the ad- 
vantages of wide-band reception are obvious. 

Simplicity of Equipment 

A further advantage arises in the reduced 
amount of equipment required. Not only can 
listening gear be made with less than half the 
bulk of echo-ranging gear but, perhaps even 
more important, it requires only a small frac- 
tion of the power to operate. This, of course, 
is of particular interest in the case of small 
craft and submarines where space and efficien- 
cy are primary considerations. 

FACTORS AFFECTING 
PERFORMANCE 

Detection of a target by listening is a com- 
plex matter which depends not only on the 
original characteristics of the noise signal but 
upon the transformations and losses suffered 


during transmission through both the sea and 
the acoustic system employed. It is also highly 
dependent upon the ability of the operator to 
recognize and identify characteristic sounds 
against a masking beckground which is in large 
measure uncontrollable. 

The sea is by no means a homogenous, iso- 
tropic medium. Variations in temperature, 
salinity, ocean bottom, and surface all affect 
the propagation of sound. Transmission losses 
result both from the spreading of the sound 
intensity and from attenuation due to scatter- 
ing and absorption. Reflections from the sur- 
face and bottom introduce phase errors, while 
refractions, caused principally by temperature 
gradients in the water, may bend the sound 
path so severely that reception is impossible 
except at extremely short ranges. Ambient 
noise present in the water, distinct from the 
self-noise of the ship and listening equipment, 
may be caused by surface conditions, marine 
life, or other ships. In coastal waters, man- 
made noises may reach high levels.*" 

The problem confronting the designer of 
equipment for underwater listening has two 
main aspects: first, the extension of the effec- 
tive range of initial detection and identifica- 
tion of the target; second, the improvement of 
the accuracy of bearing determinations. 

Extension of Range 

Maximum range for a listening system is al- 
ways determined by the signal-to-noise ratio. 
Consequently, extensions of range can only 
be accomplished by increasing this ratio. As- 
suming a fixed signal at the hydrophone, this 
can be done only by lowering the noise. Ampli- 
fiers and hydrophones now available have made 
it possible to eliminate electric noise as a factor. 
Self-noise, generated by the vessel’s machinery 
and motion through the water, can be greatly 
reduced by the use of antivibration mountings, 
acoustic insulation methods, and proper stream- 
lining. Although such measures have not yet 
been fully exploited, they have succeeded in 
reducing the importance of self-noise as a 

^ These basic factors relating to the behavior of 
sound in the sea have been extensively investigated 
and are discussed in detail in Division 6, Volumes 7 
through 9. They are also reviewed briefly in Chapter 3. 




4 


INTRODUCTION TO LISTENING 


factor in submarine installations for all except 
the higher submerged speeds. Self-noise con- 
tinues to be the limiting factor for sonic fre- 
quencies in surface craft except for very low 
speeds. 

Under conditions of low self-noise, signal-to- 
noise ratios are determined by the background 
noise present in the water. The effects of such 
random noise can usually be reduced by mak- 
ing the hydrophone more directional, so that 
it responds only to sound coming from a nar- 
row angle. The two factors that impose a limit 
on improvement from this direction are the 
dimensions of the hydrophone and the useless- 
ness of a needle-sharp beam for search opera- 
tion. 

A study of the problems of extension of 
range and improvement in bearing accuracy 
leads to a consideration of the directivity of 
the hydrophone. There are many arguments 
for making the hydrophone very directive. Not 
only is the signal-to-noise ratio improved by 
the exclusion of random noise from all direc- 
tions but also it is obviously easier to determine 
direction accurately if a narrow-beam pattern 
is provided. On the other hand the provision 
of a too narrow beam slows up the rate at 
which a given sector of ocean can be scanned. 
A compromise must therefore be drawn be- 
tween extension of range and scanning rate. 

Improvement of Bearing Accuracy 

Improvement of bearing accuracy is essen- 
tially an engineering problem and is largely 
dependent upon the beam pattern of the hydro- 
phone : beam width, directivity index, side 
lobes, and rear response. All these are, in turn 
dependent upon the structure of the hydro- 
phone and the frequency of incident signals. 
Steering may be accomplished by mechanical 
training or electrically by introducing ap- 
propriate phase lags between members of an 
array of hydrophones. Bearing accuracy has 
been further increased by the technique of 
split pattern hydrophones in which the direc- 
tion of maximum signal is determined by com- 
paring the response of two separate patterns 
and discriminating electrically on the basis of 
phase or amplitude differences between the 
two. Such systems can be made very sensitive 


and have the additional feature of indicating 
the direction of error in bearing.’" 

In Chapter 6 of this volume, attention is 
given to the phase-actuated locator [PAL] and 
the right-left indicator [RLI] . 

13 ANTISUBMARINE APPLICATIONS 

Patrol Craft Systems 

The first application of listening gear by 
Division 6 to a tactical problem was dictated 
by urgency. The years 1942 and 1943 were 
marked by a sharp upsurge in U-boat activity 
against our Navy and Merchant Marine. With 
few sonar-equipped antisubmarine vessels avail- 
able for patrol and convoy work, hasty efforts 
were made to develop a simple listening system 
that would enable small converted civilian 
craft to assist in coastal patrol. It was felt that 
although such craft were not equipped to at- 
tack and destroy submarines, they could detect 
and localize the targets until help could be 
summoned. Later, the Navy found it possible 
to provide ever increasing numbers of better- 
equipped military craft. Consequently, the 
oversimplified listening gear produced for this 
purpose was not put into actual service. 

The information gained in this program, 
however, was extremely valuable and became 
applicable to all sonar designs. Significant 
contributions were made to the analysis of 
typical target noises and the evaluation of the 
effectiveness of various design parameters of 
the detecting gear. Chapters 4 and 5 of this 
volume review some of this basic work. 

The Radio Sono Buoys 

At present it is generally true that echo 
ranging is more effective in the detection of 
submarines than is direct listening. As pointed 
out previously, this results from the extreme 
quietness of the modern submarine when sub- 
merged. It also is a result of the very high self- 
noise in the useful sonic region of destroyers 
at patrol or attack speeds. The principal con- 
tribution of listening gear in the field of anti- 


^ The reader is referred to Division 6, Volume 15, 
for a general discussion of bearing deviation indicator 
[BDI] systems. 


THE PROSUBMARINE PROGRAM 


5 


submarine warfare, the radio sono buoy, avoids 
this difficulty. 

Essentially, the radio sono buoy is a small, 
rugged, sonic listening system which is capable 
of transmitting by radio the underwater 
sounds it picks up. As the receiver can be re- 
motely located either in an aircraft or a sur- 
face ship and can be instantly tuned to any 
one of a number of buoys, it makes possible 
the detection and tracking of submarines with- 
in the range of its hydrophone. By removing 
the noise of a searching ship, the ranges ob- 
tainable with these buoys are often consider- 
able, since the submarine, having no immediate 
cause for suspicion, is not rigged for silent 
running. Later versions of these buoys added 
the feature of directivity (directional radio 
sono buoy [DRSB]) and thus not only ex- 
tended their range but also provided with a 
single buoy a geographical bearing of the sub- 
marine's position. 

These buoys found their principal use in 
permitting aircraft to establish contact with 
submerged U-boats. They were extremely suc- 
cessful in this application. A later application 
of great value was in their use in the last 
days of the war by surface craft in the 
English channel against U-boats operating 
with Schnorkel. 

' ‘ THE PROSUBMARINE PROGRAM 

Because of the nature of the several factors 
involved, an effective application of listening 
gear is in the field of prosubmarine activity. 
A submarine is a weapon of stealth and must, 
to maintain its usefulness, operate unobserved 
and undetected. On the other hand, surface 
craft, the submarine’s natural enemies and 
prey, are as a class noisy and therefore ideal 
targets for a sonic listening system. Further, 
during the submerged evasion period when the 
submarine is most helpless, the sonic contact 
it is able to establish is its one means of obtain- 
ing information concerning the number, dis- 
position, and intent of its attackers. 

A typical submarine sonar installation in- 
cludes equipment for echo sounding to deter- 
mine depth of bottom, for echo ranging to de- 
termine precise target and minefield location, 
for supersonic listening, usually utilizing part 


of the echo-ranging gear, and for sonic listen- 
ing, usually from hull or topside-mounted gear 
for general-purpose use. 

Use of Echo Ranging 

The submarine uses its echo-ranging gear 
for navigation, locating channels, charting 
minefields, and operating under conditions 
when other aids fail. Under wartime condi- 
tions in enemy waters or in the neighborhood 
of enemy craft when security is paramount, 
its use is avoided. An exception is made only 
when, for purposes of obtaining a final range 
on a target before launching a torpedo, a single 
ping is emitted. The reason for this is dem- 
onstrated by Figure 1, which shows that a typi- 



Figure 1. Total transmission loss for echo rang- 
ing and listening signals with range. 


cal condition, giving a maximum echo-range of 
1,000 yards, permits the detection of the pulse 
transmission at a range of over 10,000 yards 
by an enemy alert in that direction. 

Use of Listening 

Listening gear, on the other hand, is in al- 
most constant use during submerged patrol and 
attack operations. In the Pacific war against 
the Japanese, our submarines found it desir- 
able to make full use of radar and the periscope 
for long- and medium-range detection. This 
was possible, however, only because of the in- 
adequacy and poor design of Japanese radar. 
It cannot be assumed that this condition will 
obtain with any future enemy. Below peris- 
cope and radar depth and at short ranges, 
reliance must be placed on listening. 


6 


INTRODUCTION TO LISTENING 


To the uninitiated, the amount of informa- 
tion that can be secured by a trained operator 
with efficient listening gear is remarkable. 
Ships can not only be detected but also can 
be identified as merchantmen, destroyers, or 
battleships at ranges of thousands of yards. 
An extreme range of 42,000 yards (21 nautical 
miles) was verified on a battleship. Further, it 
has been demonstrated that bearing accuracies 
of better than 0.25 degree can be consistently 
realized by using split hydrophone techniques. 
Such performance has been made possible in 
this country only during the course of World 
War II and is ascribable to several develop- 
ments. The first of these is the use of sonic 
frequencies. Supersonic listening gear, already 
installed in many submarines before the war, 
was preferred because of the lower noise back- 
ground levels in the higher frequencies and be- 
cause of the large hydrophone dimensions re- 
quired for good directivity at the lower sonic 
frequencies. Little was known of the character 
of target noise or of the attenuation character- 
istics of water. As information was acquired 
on these subjects and the usefulness of the 
sonic band for maximum range and target 
identification was realized, a determined effort 
was made to develop an adequate hydrophone. 

The line hydrophone, the second major de- 
velopment, made possible the reception of sonic 
frequencies with good directivity, which is the 
measure of the ability of a hydrophone to dis- 
criminate against noise. Built in the form of 
a long, small-diameter tube, the line hydro- 
phone, when mounted, is highly directional in 
a horizontal plane and nondirectional in a 
vertical plane. This latter pattern is con- 
sidered desirable, since it permits following a 
target having a high vertical angle. 

Finally, improved amplifiers utilizing the 
favorable signal-to-noise and wide-band char- 
acteristics of the line hydrophone provided 
ideal reproduction of sounds over the entire 


sonic region and, by means of a heterodyne 
converter, over the supersonic region. With 
split hydrophones, specialized circuits were 
able to furnish highly accurate bearing indi- 
cations by comparing electrically the time of 
arrival of the signal at each of the two hydro- 
phone sections. 

A most useful application of listening is that 
of torpedo detection. This was accomplished 
by the simple expedient of plotting the ampli- 
fied output of a continuously rotated hydro- 
phone on a bearing recorder. Not only can the 
torpedo be detected almost at the instant of 
launching but also it is possible to tell whether 
the torpedo’s course leads, lags, or intercepts 
the course of the listening ship. 

Aircraft Detection 

A task yet to be solved is the detection of 
enemy aircraft in the vicinity above the sub- 
merged submarines. Had such equipment been 
available to the Germans in the last years of 
the war, it might have saved the majority of 
their submarines sunk by radar-equipped air- 
craft. Because, with present armament, the sur- 
facing submarine is helpless for an appreciable 
period, the advantage lies with high-speed at- 
tacking aircraft. Preliminary studies have 
shown the infeasibility of detecting the noise 
of aircraft under water at any useful range. 
This is due, primarily, to the very high losses 
airborne sound suffers when entering water. 
It is suggested, however, that the principle of 
the sono buoy, inverted to serve as an acoustic 
transformer from air to water, may provide a 
possible solution to this problem. 

Succeeding chapters of this volume review 
the evolution of listening systems, discuss typi- 
cal experimental techniques and data, and, in 
the final chapters, present a detailed report of 
the principal equipment developments accom- 
plished by Division 6 laboratories. 




Chapter 2 

EVOLUTION OF LISTENING GEAR 


^ EARLY SONIC BEACON 

Underwater Bells 

U NDERWATER SOUND waves Were employed 
as navigation aids for many years prior 
to World War I. During this period a con- 
siderable number of underwater bells were 
installed as part of coastal beacon systems. 
The sound waves emitted by these bells were 
picked up by electroacoustic devices which 
used a metallic diaphragm equipped with an 
inertia-type carbon button microphone. The 
diaphragm was, in general, responsive only to 
a limited range of frequencies, including the 
characteristic frequency of the bell to be de- 
tected. Hydrophones of this type were generally 
mounted in pairs in tanks located in the fore 
peak of the vessel, one on either side of the 
bow. Some indication of the direction of the 
bell could, therefore, be obtained by comparing 
the relative response of the two units. 

The Fessenden Oscillator 

Immediately prior to World War I, produc- 
tion designs had been completed on the Fessen- 
den oscillator which was intended to provide 
a more effective source of underwater sound 
than the bells which had previously been used. 
The Fessenden oscillator consisted of a struc- 
ture weighing several hundred pounds and 
having a heavy diaphragm resonant at approxi- 
mately 500 c. This diaphragm was driven elec- 
tromagnetically, the coupling being effected 
through eddy currents set up in the diaphragm 
itself. The electric input to the Fessenden oscil- 
lator was obtained from a motor-driven alter- 
nator. Experience with the oscillator demon- 
strated that it could be used for reception as 
well as for transmission of underwater sound 
waves having frequencies near that to which 
it was tuned. For such reception it was neces- 
sary merely to provide a proper polarizing cur- 
rent and to connect a conventional telephone 
receiver, or headset, to the oscillator terminals. 


For normal navigation use, it was intended 
that vessels should be equipped with carbon 
button hydrophones tuned to the oscillator fre- 
quency. The oscillators were to be mounted at 
designated points along the coast where they 
would serve as sonic beacons. 


WORLD WAR I SYSTEMS 

Systems using either the submarine bell or 
the Fessenden oscillator may properly be classed 
as communication systems, inasmuch as they 
include transmitting stations capable of send- 
ing signals based upon a prearranged code. 

It was discovered that either the carbon 
button hydrophone or the Fessenden oscillator, 
used as a receiving device, was capable of pick- 
ing up sounds which might be present in the 
water due to natural causes. Sound due to the 
propellers or to the machinery of ships, for 
example, could be detected, provided these 
sounds contained components having frequen- 
cies within the narrow band to which the 
response of the hydrophones was restricted. 
The first step toward the detection of moving 
vessels by means of underwater sound waves, 
therefore, had been taken prior to 1917. 

Immediately upon our entry into World War 
I, attention was directed to the further devel- 
opments of underwater acoustic devices as pos- 
sible means of defense against enemy sub- 
marines. All U. S. submarines were equipped 
with Fessenden oscillators to enable them to 
signal one another while submerged. It was 
realized, however, that the limited range of fre- 
quencies to which the oscillator was responsive 
seriously limited its usefulness in listening for 
surface vessels. Superior detection devices with 
a wider range of frequencies were, therefore, 
diligently sought. 

It must be remembered that during this 
period the vacuum-tube amplifier had not yet 
come into general use and that the efficiency of 
the pickup device itself determined its effective- 
ness to a far greater degree than is today the 




7 


8 


EVOLUTION OF LISTENING GEAR 


case. As a result, much of the early effort was 
directed toward obtaining hydrophones the 
electric outputs of which were of sufficient 
magnitude to operate telephone receivers di- 
rectly. The carbon button microphone con- 
tinued to be used for a long time even though 
its inherent electric noise, as shown by sub- 
sequent measurements, is significantly above 
the level of its response to normal background 
noises. 

Magnetophone 

Toward the close of World War I, the devel- 
opment of electronic amplification had reached 
a point where hydrophones having both lower 
electric response to acoustic waves and lower 
internal circuit noise could be used advanta- 
geously. One such hydrophone, known as a 
magnetophone, employed a conventional tele- 
phone receiver as the electroacoustic trans- 
ducer. This receiver was coupled acoustically 
to the water by means of a closed rubber tube. 
The magnetophone was capable of detecting 
both surface vessels and submarines at con- 
siderable distances. Its performance was not, 
however, enough better than that of the carbon 
hydrophone to justify the difficulties then 
attending the use of vacuum tubes. 

Acoustic Systems 

Concurrently with the development of elec- 
troacoustic devices, considerable success was 
achieved in the use of purely acoustic listening 
methods. A metal tube, for example, closed at 
the lower end by a short length of rubber 
tubing, was found to bring underwater sounds 
directly to the ear of a listener almost as ef- 
fectively as the electroacoustic devices. Such 
tubes were in use on our submarines until some 
time after our entry into World War II. 

Towed Systems 

Even during early attempts at underwater 
listening, the electroacoustic devices exhibited 
one marked advantage over the simpler acoustic 
arrangements. The latter are seriously limited 
by the high level of background noise resulting 


from their mechanical contact with the listen- 
ing vessel. Noises due to ship's machinery, to 
the activities of the crew, and to waves break- 
ing against the hull all interfere with the re- 
ception of sound waves from a distant vessel. 
With the electroacoustic devices, however, it 
was possible to support the unit at a distance 
from the ship by means of suitable buoys and 
to carry the electric signals to the ship by 
means of a cable. This system had its obvious 
disadvantage in operation. Noises due to motion 
of the hydrophone through the water and to 
the listening vessel constituted an effective 
barrier to the reception of faint signals. 

Frequency Coverage 

The best of the early listening devices, 
although markedly superior to the prewar 
resonant devices, nevertheless were responsive 
over a frequency range which was by no means 
extensive as compared with present-day elec- 
troacoustic systems. It is doubtful that these 
early hydrophones showed significant response 
to frequencies below 300 c or above 2,500 c. 
Over this region the response was far from 
uniform but passed through a maximum some- 
where between 500 and 1,000 c. 

Throughout the entire period of World War 
I, the development of listening systems was 
carried on without the benefit of reliable data 
on the frequency distribution of energy in 
sound waves set up by ships or in the noises 
interfering with their reception. Attempts 
were made to obtain such data but these barely 
reached the point of determining the response 
characteristics of the measuring equipment. 
They contributed virtually nothing toward the 
discovery of the optimum frequency or band of 
frequencies to be used for the detection of 
significant signals. 


Bearing Determinations 

Attention was next given to the possibility 
of determining bearing, or the direction of 
origin of underwater sounds. Two important 
contributions to the art were made. One was 
the so-called binaural method of listening, the 
other was the multispot array. 




WORLD WAR I SYSTEMS 


Binaural Systems 

The first binaural system was purely acoustic. 
Two metal tubes were carried by a framework 
supported over the side of a listening vessel 
in such a way that the closed rubber termi- 
nating tubes which were separated by a hori- 
zontal distance of approximately 4 feet could 
be rotated in a horizontal plane. One tube led 
to each ear of the observer. As the tubes were 
rotated, it was observed that the apparent loca- 
tion of the signal source moved relative to the 
observer. When the line joining the two rubber 
terminating tubes was perpendicular to the 
direction of the source, the apparent location 
seemed to be directly in front of, or directly 
behind, the observer. The identification of posi- 
tion involved the interpretation of subjective 
sensations and required a considerable amount 
of practice. There are two bearings, differing 
by 180 degrees, for which the characteristic 
zero-position effect may be obtained. The true 
bearing of the source, however, may be identi- 
fied by rotating the binaural pair away from its 
position perpendicular to the actual bearing 
and observing the direction in which the acous- 
tic image appears to move. 

Performance. The use of binaural tubes per- 
mitted the determination of bearing with rea- 
sonable reliability and thereby increased the 
utility of underwater sound in obtaining in- 
formation regarding the position and move- 
ments of invisible enemy craft. Actually, the 
use of the binaural system did more than 
permit the determination of bearing: it in- 
creased the discrimination of the listening sys- 
tem for sounds from a localized source and 
helped to distinguish them from interfering 
random noise. A faint signal arriving along a 
definite bearing could be identified by its ap- 
parent movement when its relative intensity as 
compared with the intensities of interfering 
sounds was so low that it could not be rec- 
ognized by a nondirectional system. In terms 
now familiar, the limiting signal-to-noise ratio 
for a binaural system is lower than for a simple 
nondirectional system. It must be emphasized, 
however, that this effective signal-to-noise ad- 
vantage depends upon psychoacoustic sensa- 
tions rather than upon measurable apparatus 
characteristics. It is, however, a very real ad- 


vantage analogous to our ability to locate a 
squirrel in a tree more easily if he moves than 
if he remains motionless. 

Electroacoustic Application. The capabilities 
of binaural listening proved to be so superior 
to those of simple nondirectional systems that 
the basic technique was applied to electro- 
acoustic devices. Two hydrophones were 
mounted at a fixed separation and in fixed posi- 
tion and orientation in the water. Each hydro- 
phone was connected by cable to a specially 
designed telephone receiver. The acoustic out- 
put of these, in turn, was led individually to 
the ears of the observer by means of ducts of 
adjustable lengths. By means of a calibrated 
control wheel, the lengths of these ducts were 
changed to give the same phase relations be- 
tween the two signals as occurs in the simple 
acoustic system when the pickup units are set 
perpendicular to the bearing of the source. The 
acoustic phase shift device was known as a 
binaural compensator. The directional ambi- 
guity which obviously appears in this case, as 
well as in the case previously described, was 
resolved by observing the indicated bearings 
obtained with two or more binaural pairs set in 
different orientations. Bearings which reap- 
peared consistently were taken as the correct 
bearing. 

Multispot Arrays 

The second method for the determination of 
bearing, namely, the multispot array, depends 
upon the fact that the magnitude of the re- 
sponse of the system is a function of the 
relative bearing of the origin of the acoustic 
wave. The determination of bearing is effected 
l3y adjusting the position of the multispot array 
itself or by altering the phase relations of ele- 
ments of the system so as to make the aggre- 
gate response to some particular sound a maxi- 
mum. This work reached the point of establish- 
ing the great advantage of a directional listen- 
ing device operating on the maximum intensity 
principle as a means for improving signal-to- 
noise ratio. It is important to note that, in 
spite of the improvements in hydrophones and 
in the use of electronic amplification achieved 
in recent years, the detection ranges possible 
with these early installations compared favor- 


10 


EVOLUTION OF LISTENING GEAR 


ably with the best detection ranges obtainable 
during World War II. One reason for this 
somewhat surprising fact is that the newer and 
more effective elements were never combined to 
form a multispot array having the acoustic 
dimensions of those used during World War I. 

Military Applications 
Antisubmarine Gear 

The status of direct listening throughout the 
entire period of World War I may be sum- 
marized briefly. Submarines of that period 
were not particularly quiet. Most of those in 
operation had been planned, if not constructed, 
prior to the development of underwater listen- 
ing equipment, hence there was no particular 
incentive to make them quiet. As soon as the 
potential capabilities of direct listening were 
appreciated, steps were taken to reduce the 
noise of our own submarines. 

The first important application of direct 
listening was made from surface craft. It must 
be remembered that throughout World War I 
there was no effective method for the accurate 
determination of the range of a submerged sub- 
marine. The nearest approach to this essential 
information with the facilities then available 
was to use two or more vessels equipped with 
directional listening devices and to estimate the 
position and movements of the submarine on 
the basis of crossed bearing intersections. This 
arrangement was feasible because it required 
little movement on the part of the listening 
vessels, which was the only condition under 
which the gear of that time could be operated 
effectively in any case. 

All these factors were reflected in the listen- 
ing gear in use by surface vessels at the end 
of World War I. Three hydrophones were ar- 
ranged to form three binaural pairs, the axes 
of which were spaced at 120-degree intervals. 
This system was carried to a distance from the 
listening vessel by means of a buoy and cable 
rigging. Proper orientation of the system de- 
pended upon such motion through the water 
as a ship lying to may possess due to wind 
pressure. Obviously, no great bearing accuracy 
could be expected under such conditions. 

There was, throughout this period, almost no 
use made of listening gear by our own sub- 


marines. The absence of both radar and under- 
water echo ranging permitted the submariner 
to obtain practically all the information needed 
for an attack by means of the periscope. 

Harbor Protection 

A second important application of direct 
listening methods during World War I was in 
harbor protection. Here multiple binaural pairs 
similar to those described above were mounted 
on tripods planted on the ocean bottom at 
suitable locations near harbor entrance chan- 
nels. These were connected to shore stations by 
electric cables. Provision was made for select- 
ing any desired binaural pair by means of 
relays operated by pulses transmitted over the 
listening cables. Because of the accurately 
known positions of the tripods and because of 
the possibility of calibrating the orientation by 
means of a source having an accurately estab- 
lished position, these tripod systems were capa- 
ble of great reliability in the determination of 
the position of any sound source within their 
range. 

Listening Underway — Multispot Arrays 

The third important experimental develop- 
ment in direct listening which took place 
during World War I was in connection with 
multispot arrays. Models were built in which 
the individual units were carbon button hydro- 
phones. These were connected independently to 
specially designed telephone receivers similar 
to those in the electroacoustic binaural systems. 
As in those systems the acoustic outputs were 
combined by ducts of adjustable lengths where- 
by the desired phase delays were introduced. 
The purpose in mind in developing these multi- 
spot arrays was to see whether sufficient im- 
provement in signal-to-noise ratio might be ob- 
tained to permit operation from a vessel under 
way at moderate speed. 

2 3 DIRECT LISTENING BETWEEN 

WORLD WARS I AND II 

The period between World War I and World 
War II witnessed marked development of 
sonar equipment and methods. Supersonic echo- 
ranging equipment was one of the chief new 


DEVELOPMENTS DURING WORLD WAR II 


11 


devices. The use of high frequencies made pos- 
sible the construction of highly directive trans- 
ducers of such size that they could be mounted 
and trained from a vessel moving through the 
water at considerable speed. The directional 
properties of these transducers gave adequate 
discrimination against locally generated noises 
so that echo signals could be received over suf- 
ficient range to have tactical utility for both 
search and attack operations. 

The experience of World War I led to the 
designing of submarines capable of operating 
with very little noise. In fact, the range at 
which a submerged submarine might be de- 
tected with the best ship-mounted direct listen- 
ing gear came, in time, to be considerably less 
than the range of detection possible by echo- 
ranging equipment. 


Supersonic Listening 

Finally, it was found that the highly direc- 
tive echo-ranging projector could also be used 
for direct listening by heterodyning the re- 
ceived supersonic signals to produce a signal 
in the audible portion of the spectrum. Such 
direct listening in the supersonic region is, of 
course, subject to the same limitations with 
respect to signal-to-noise ratio as apply in the 
audible region although, in general, back- 
ground noise is much reduced at higher fre- 
quencies. The directivity of the projector, how- 
ever, gave a considerable advantage to opera- 
tion at high frequencies. Comparable directivity 
at audio frequencies would have demanded 
transducers of prohibitive size, although it 
could have been obtained by multispot arrays. 

AttcTiiidtioTi. Attenuation is one adverse 
aspect to direct listening at frequencies above 
the audible portion of the acoustic spectrum as 
the attenuation factor increases with frequency. 
This is of little significance at short distances 
but is of major importance whenever detection 
over long ranges is sought. Acoustic waves of 
high and of low frequency suffer about the 
same transmission loss in traveling a thousand 
yards from their source. At a range of 10,000 
yards, however, a wave having a frequency of 
30 kc is reduced in energy intensity some 
650,000 times as much (58 db) as a 1-kc wave. 


Further Development of Multispot Arrays 

During the interval between the two World 
Wars, the multispot array was further devel- 
oped in two ways. One was the use, in place of 
acoustic delay ducts, of electric delay networks 
and the introduction of the proper phase shifts 
directly into the electric outputs of the several 
hydrophone units. These outputs were then 
combined to form a single electric signal which 
could be made to show a maximum for a sound 
source on any desired bearing, depending upon 
the adjustment of the delay networks. The 
second improvement was the construction of 
harbor protection units for operation from 
fixed stations. These units were of considerable 
size, weighing approximately 16 tons and hav- 
ing a total span of approximately 50 feet. The 
array was divided into two 25-foot units. Each 
half was 4 wavelengths long at 800 c. It con- 
sequently had a high degree of directivity. The 
two 25-foot arrays were mounted along a single 
line which could be rotated in a horizontal 
plane. The hydrophones were so connected to 
telephone receivers at the shore station that 
each array functioned as one element of a bin- 
aural pair. The system as a whole, therefore, 
combined the objective signal-to-noise advan- 
tages of a maximum-intensity directional 
device with the subjective advantages of 
binaural operation. An installation arranged in 
this manner gives almost ideal direct listening. 


DEVELOPMENTS DURING 
WORLD WAR II 

With the renewed interest in underwater 
sound brought about by the imminence of a 
second world war, development of new devices 
in the field of direct listening was again actively 
resumed. At this time, however, the science of 
acoustics had reached a point where work could 
be carried forward on a firm quantitative basis 
rather than by trial and error. The availability 
of accurate standards for the measurement of 
the intensities and behavior of acoustic waves 
in water and for the determination of the per- 
formance characteristics of listening devices 
contributed to rapid advance. 


12 


EVOLUTION OF LISTENING GEAR 


Studies to Determine Optimum 
Frequency Characteristics 

One of the first steps in the renewed program 
was an empirical determination of the general 
relations between the frequency characteristic 
of a given listening system and its ability to 
detect a faint signal against interfering noises. 
The first observations were made with com- 
pletely nondirectional systems. The ranges were 
compared at which sounds due to various types 
of ships could be identified through represen- 
tative background noises by an observer listen- 
ing on a pair of high-quality telephone re- 
ceivers. It was noted that each observer showed 
an initial tendency to prefer systems respon- 
sive over the lower portion of the frequency 
spectrum, that is, to frequencies in the range 
below about 3,000 c. This tendency appeared 
to result from preconceived ideas of natural- 
ness and it was interpreted to indicate that the 
sounds heard agreed with those which the ob- 
server expected to hear on the basis of his 
previous experience with airborne sounds as- 
sociated with the movement of objects in water. 

It was soon realized, however, that fidelity, 
as understood in connection with the high- 
quality reproduction of sounds by radio or 
phonograph, meant little with respect to the 
detection of underwater signals, particularly 
at low levels. Once this was recognized, it be- 
came evident that audio components having 
frequencies above 3,000 c were of great value. 
In fact, systems responsive to frequencies as 
high as 10,000 c appeared more promising than 
systems incapable of responding to these fre- 
quencies. It appeared to be advantageous under 
many conditions to exclude components having 
frequencies at the lower end of audible range. 
The conclusions based on these tests, which 
were of a purely subjective nature, were in 
agreement with actual measurements of the 
frequency characteristics of underwater sounds 
which indicated that the energy associated with 
ship sounds fell off with frequency less rapidly 
than does the energy associated with water 
noises due to waves, surf, and other causes. 

It must be kept in mind that the character 
of the sound and the ability of the ear to rec- 
ognize this character played an important 
part in fixing the actual relative level of signal 


with respect to noise at which detection became 
possible. Where detection depends solely upon 
the ability of some instrument to indicate a 
discernible change in the magnitude of its total 
reading, the change being identified with the 
source to be detected, it is probable that there 
would be found to exist a narrow band of fre- 
quencies, or even a single frequency, for which 
this change showed a maximum fractional 
value. Such a single frequency, however, would 
certainly depend upon the particular conditions 
existing at the time of the trial and would vary 
greatly with changing circumstances. The 
ability of the ear to recognize some identifying 
characteristic in a given signal permits the 
detection of this signal while at a much lower 
relative level with respect to interference than 
would be possible with any instrument measur- 
ing only total energy. This situation finds a 
familiar analogue in our ability to hear spoken 
words against a background of general noise 
even though the energy associated with the 
noise may be many times that associated with 
the speech. In the case of ship sounds, the 
rhythm of the propeller beat often supplies the 
identifying characteristic. To permit the ear to 
recognize a given signal as a separate entity, 
it is necessary that the receiving system be 
responsive over some appreciable band of fre- 
quencies. This appears to be true even though 
the resultant signal-to-noise ratio, as measured 
in terms of absolute energy, is less than would 
be the case with a more restricted band. The 
ear itself is capable of applying selective dis- 
crimination to the several components making 
up the total signal delivered by the listening 
system. 

Electroacoustic Transducer Status 

The conclusions reached as a result of the 
experiments described above would have been 
of little practical value had it not been that the 
art of constructing electroacoustic transducers 
had reached a point where the electric response 
to the lowest acoustic levels encountered ex- 
ceeded the electric circuit noise. These trans- 
ducers used the piezoelectric properties of 
plates of crystalline material or the magneto- 
strictive properties of certain metals. The 



APPLICATIONS IN WORLD WAR II 


13 


mechanical constants of these materials pro- 
vide better electroacoustic coupling to water 
than is the case for transducers designed for use 
with airborne sounds. Although there are many 
differences between magnetostriction trans- 
ducers and piezoelectric transducers, these con- 
cern design details rather than overall per- 
formance. Quantitative knowledge regarding 
both types is now so complete that almost any 
given combination of operating requirements 
may be met with equal success by either. 

With these hydrophones detection is limited 
solely by signal-to-noise ratio rather than by 
actual sensitivity. The maximum range at 
which a given sound source may first be dis- 
covered is, however, not determined uniquely 
by this ratio but depends also upon the ar- 
rangement and operation of the system and the 
manner in which the ultimate indication is 
presented. Several methods have been used to 
improve the detectability of a signal which 
must be observed against a high level of inter- 
ference. 

Possible Methods of Improving Detection 

The signal-to-noise ratio at which detection 
ceases to be possible may be reduced by the 
proper choice of the position and breadth of the 
listening frequency band. The ability of the ear 
to recognize a signal having some identifying 
characteristic likewise permits a reduction of 
this limiting signal-to-noise ratio. The use of 
a directive transducer in place of a nondirec- 
tional unit is accompanied by an effective im- 
provement in signal-to-noise ratio. This, of 
course, results directly from the fact that with 
a directional receiver the signal is required to 
compete only with interfering noises arriving 
along the same bearing instead of with noises 
arriving from all directions. Further, if a 
directional transducer is trained squarely on 
the bearing of an approaching sound source, 
the signal-to-noise ratio obviously increases 
slowly and gradually. Such a signal is not 
noticed by a listener at as low a level as is a 
signal which stops or, even better, which starts 
abruptly. This effect can be obtained under the 
complete control of the listener by training a 
directive transducer across the bearing of the 
sound. 


Finally there is the use of so-called crossed 
lobes. This is basically a method for employing 
a directional receiving system to improve bear- 
ing accuracy and at the same time to obtain 
information as to the sign of any deviation be- 
tween the significant axis of the system and the 
actual bearing of a sound source. It requires 
two directional transducers or their equivalent, 
so associated that the two axes of maximum 
response are separated by a small angle. The 
indicating system is then arranged to report in 
sortie suitable manner the relative response to 
a given signal as received over the two paths. 
In particular, the bearing of a source in which 
the two responses are identical may be thus 
identified with far greater accuracy than the 
bearing of maximum response of either alone. 
This enhanced bearing discrimination is ac- 
companied by an increase in the capability of 
the system to detect a faint sound when the 
critical axis is swept across the bearing of the 
sound at a suitable rate. The crossed bearing 
technique is of little if any value when the 
transducer system is maintained constantly on 
the bearing of an approaching sound source. 

2^ APPLICATIONS IN WORLD WAR II 

Changes in combat methods and the develop- 
ment of new weapons during World War II 
are reflected in the differences between the per- 
formance characteristics of direct listening 
gear now used and that of World War 1. 

Radio Sono Buoys 

An important innovation is the use of radar- 
equipped aircraft for search and attack in anti- 
submarine warfare. Once the target submarine 
is below the surface, however, radar is unable 
to supply further information as to its position 
and movements and it is frequently too late to 
obtain visual information of sufficient accuracy 
to permit reliable placement of depth charges. 
The expendable radio sono buoy [ERSB] pro- 
vided a means whereby the necessary informa- 
tion could be obtained through the use of under- 
water sound. In this device, signals from the 
buoy to the listening station are carried by 
radio transmission and thus, by removing the 
necessity of maintaining mechanical contact. 


14 


EVOLUTION OF LISTENING GEAR 


greatly extend the separation possible. Since 
direct mechanical control of the unit is not 
possible after launching, the earlier models 
were provided with a nondirectional hydro- 
phone in order that coverage of all bearings 
might be assured. This, however, reduces the 
signal-to-noise ratio below that which would be 
possible with a directional unit. 

It has been common experience that the noise 
level of a free-floating radio sono buoy is ap- 
preciably lower than when anchored in a flxed 
position where it may be subject to tidal cur- 
rents. Although the operation of the radio sono 
buoy obviously suffers no direct interference 
by acoustic noises generated by the airplane, 
severe masking interference arises from high 
noise levels inside the airplane. This situation 
can be alleviated to some degree by sound re- 
duction and insulating measures and by raising 
the observer listening level with respect to the 
noise background. With a suitably designed 
system, detection of a submerged submarine is 
possible over ranges sufficient to have tactical 
value. Estimates of the position, course, and 
speed of a submarine are possible by compari- 
son of the relative signal levels as heard over 
several units properly disposed in the vicinity 
of the target. 

Toward the close of the war, designs were 
worked out for a directional radio sono buoy 
[DRSB] containing a compass mechanism 
which, by altering the carrier frequency, per- 
mits continuous determination of the magnetic 
bearing of the transducer axis. In addition to 
the primary advantage of giving more com- 
plete and reliable information as to position, 
the directional buoy possesses greater detection 
ability than does the nondirectional form. Part 
of this improved detection ability comes from 
direct discrimination of the hydrophone and 
part from the fact that the location of the 
sound source is traversed by the directive beam 
so that the response, being of momentary dura- 
tion, is more conspicuous than if it were re- 
ceived without interruption. 

Line Hydrophones 

One of the more important developments 
relating to direct listening to be completed 
during World War II was the so-called line 


hydrophone. This was made available both in 
magnetostriction and in piezoelectric types. 
Line hydrophones exhibit maximum response 
to sources located on any bearing lying in a 
plane perpendicular to their mechanical axis. 
Directivity patterns taken in any plane passing 
through this mechanical axis, however, show a 
marked decrease in sensitivity as the bearing 
of a sound source departs from the perpendic- 
ular. Although a hydrophone in which maxi- 
mum response is limited to a plane only does 
not have so effective discrimination against 
background noise as one in which it is restricted 
to a single line, these directivity characteristics 
have certain practical advantages. The chief of 
these results from the fact that a line hydro- 
phone, mounted with its axis in the horizontal 
plane, is not so liable to lose contact with a 
target with change of depth as is a hydrophone 
having discrimination on all bearings off its 
acoustic axis. 

Applications. A number of important appli- 
cations of the line hydrophone were made dur- 
ing World War 11. Perhaps the most important 
was in providing sonic listening for our own 
submarines. Prior to the installation of line 
hydrophones as topside listening units, the only 
direct listening available to our submarines 
was by the supersonic echo-ranging equipment 
or by C tubes, simple acoustic pickup units ar- 
ranged for binaural listening. These, it may be 
recalled, were developed during World War I 
and had been installed as replacements for the 
earlier Fessenden oscillator. 

Line hydrophones were also installed on 
patrol craft by what were known as through- 
the-hull mountings. These supported the unit 
several feet below the keel and permitted it to 
be trained in a horizontal plane by means of a 
hand wheel inside the hull. In the case of a 
surface ship, however, the background noise 
level is considerably higher than in the case of 
a submarine and it is difficult, if not impossible, 
to use the line hydrophone effectively except 
when the vessel is lying to. With the sub- 
marine, on the other hand, almost no impair- 
ment of performance occurs at any normal sub- 
merged speeds. 

Use was also made of the line hydrophone in 
connection with harbor defense installations. 
In some cases the hydrophones were mounted 


APPLICATIONS IN WORLD WAR II 


15 


on tripods and connected to shore stations by 
electric conducting cables. In other cases they 
were supported from anchored buoys contain- 
ing radio transmitters. The buoys used for this 
purpose differed from the ERSB chiefly in the 
use of lower carrier frequencies, higher trans- 
mitting powers, and batteries suitable for con- 
tinuous operation over long periods. Since the 
buoy was naturally much larger and heavier 
than the radio sono buoy, it was possible to 
employ a more effective antenna system. Many 
units, in fact, were equipped with a ground- 
loop antenna. 

Investigations of the Binaural Effect 

Although practically no use was made of 
binaural listening during World War II, its 
possibilities were not completely ignored and 
some small amount of experimental investiga- 
tion was carried out. It had been learned dur- 
ing the interval between the two wars that the 
binaural effect is related to phase differences 
between the lower-frequency components reach- 
ing the two ears and to the relative amplitudes 
of the higher-frequency components. World 
War I systems operated by virtue of the first of 
these effects, since their response was re- 
stricted to the lower end of the audible spec- 
trum. Once it is established that higher-fre- 
quency components improve the effectiveness 
of direct listening systems, it follows at once 
that they may similarly improve binaural lis- 
tening. Such systems must, however, be ar- 
ranged suitably to introduce these amplitude 
differences. In everyday listening to airborne 
sounds, amplitude differences are caused by the 
shielding effect of the head. In one underwater 
system investigated, these were obtained by 
mounting two matched line hydrophones in a 
horizontal plane, with their centers approxi- 
mately 4 feet apart, and with the angle be- 
tween the two units adjustable through some 
10 degrees. When the planes of maximum 
response of the two units made an angle of 
about 5 degrees, it was found that the binaural 
effect obtained was quite satisfactory. With 
this arrangement the separation between 
hydrophone centers introduces the desired 
phase shifts between the two signals for all 
frequencies at which this is the determining 


factor. At higher frequencies, where the bi- 
naural effect depends upon relative amplitudes, 
the off-setting of the directivity characteristics 
produces the desired effect. Such brief trials 
as were made of this system indicated, in a 
qualitative manner, that they were superior to 
systems depending solely upon the relative 
phases of signal components. This arrange- 
ment, in addition to improving the binaural 
effect, simultaneously achieves a considerable 
direct signal-to-noise advantage due to the 
directional discrimination of the two units. 

Visual Indicators and Recorders 

Torpedo Detectors. The development of vis- 
ual indicating or recording instruments in con- 
nection with systems directly responsive to 
underwater sounds has resulted in important 
practical applications. One of these has to do 
with the detection of approaching torpedoes. 
The significant performance characteristic of 
an indicator for this purpose may be called its 
differential sensitivity.^ It is particularly neces- 
sary to maintain high differential sensitivity 
over a broad spread of initial background levels 
in the case of torpedo detection. Reception is 
accomplished by rotating the standard echo- 
ranging projector at a uniform rate to provide 
coverage of all bearings. It has the advantage, 
over a nondirectional device, of an improved 
signal-to-noise ratio, characteristic of all sys- 
tems having directional discrimination, and of 
the enhanced prominence of signal increases of 
abrupt momentary duration. Experience with 
such systems, however, discloses that the back- 
ground levels encountered at various positions 
during 1 revolution of the projector may vary 
as much as 40 db over an angle not including 
the submarine’s own propellers. It is obviously 
desirable that the indicating system maintain 
high differential sensitivity at any level en- 
countered. This requirement appears to be met 
with reasonable adequacy by the cathode-ray 
tube operated as a logarithmic deflection indi- 


The differential sensitivity expresses quantitatively 
the ability of the instrument clearly to report a change 
in the amount of total energy received which, in the 
absence of some other identifying characteristic, may 
be the only evidence of the presence of an enemy 
vessel. 


16 


EVOLUTION OF LISTENING GEAR 


cator. The cathode-ray tube has the added ad- 
vantage that it may, at the same time, be so 
arranged as to indicate the bearing of any 
source causing the differential indication. De- 
velopments have been undertaken looking 
toward the utilization of the crossed-lobe prin- 
ciple for enhancing this effect further. 

Automatic Target Followers [ATF]. Auto- 
matic control of the power training mecha- 
nism is another development pertaining to indi- 
cating devices. These automatic target-follow- 
ing devices'" employ some version of the crossed 
lobe principle by which the response of the 
system may indicate the sign of any deviation 
from correct bearing and thereby properly 
apply the correct restoring control. In follow- 
ing automatically some given target, the level 
of general interfering background is, of course, 
more nearly constant than in the search pro- 
cedure used for torpedo detection. The signal 
level at which the system is most sensitive to 
incremental changes may, therefore, be ob- 
tained by manual adjustment and readjustment. 
It is, however, still desirable to have the equip- 
ment reasonably sensitive to incremental 
changes over an appreciable spread of levels. 

It has been demonstrated that devices for 
maintaining a projector automatically on the 
bearing of some target are capable of high 
bearing accuracy. This accuracy is, in fact, of 
such order that it is now possible to deter- 


^ See material pertaining to automatic target train- 
ing [ATT], Volume 15, Division 6. 


mine the range of a target vessel by crossed 
bearings from two projectors mounted at oppo- 
site ends of a submarine. The separation of the 
projectors is then used as a base line for tri- 
angulation. The anticipated development of this 
so-called triangulation-listening -ranging [TLR] 
promises to provide a most important instru- 
mental aid to our submarines. During torpedo 
approach maneuvers, the commander requires 
as complete and accurate information as pos- 
sible regarding the position of the target at any 
instant and also regarding its course and speed. 
Information as to position may be obtained 
with considerable accuracy by means of echo 
ranging. The repeated trials required to deter- 
mine the rates of change of range and bearing 
and thus to establish course and speed are, 
however, undertaken only at great risk inas- 
much as continued echo-ranging transmissions 
offer the enemy an opportunity to discover the 
position of the attacking submarine. TLR, on 
the other hand, maintains the desired continual 
flow of range and bearing data silently. The 
continuity of range and bearing data obtained 
by such a system not only provides quantitative 
information as to the rate of change of each of 
these quantities but actually further increases 
the inherent accuracy of the system by per- 
mitting statistical averages to be secured. 

In the light of current trends in subsurface 
warfare techniques, there are indications that 
the advantages of TLR, as compared with pres- 
ent echo-ranging methods, will increase in im- 
portance as time goes on. 




Chapter 3 

PHYSICAL FACTORS AFFECTING THE TARGET SIGNAL 


INTRODUCTION 

T he determination of expected perform- 
ance of any listening system is obviously 
dependent upon a knowledge of the factors 
affecting the character and strength of the 
signal to be detected by the listening hydro- 
phone. These include analysis of the noise 
emitted by the target, of the transmission 
characteristics of the conducting medium, and 
of the background noise through which the 
signal must be recognized. 

These matters, being fundamental to the 
problem of detecting and locating a submerged 
target by means of sound, have been subject to 
continuous study by Division 6 laboratories 
and investigators.* Because, however, any 
discussion of underwater listening equipment 
must be accompanied by an understanding of 
the part these factors play in its design and 
performance, they are very briefly reviewed 
in the following sections. 

NATURE OF TARGET NOISE 

The intensity and character of sound emitted 
by a target determine, under good conditions, 
how far the target can be heard. In general, 
target signal spectra depend on the type of 
ship, the number of propellers, the ship speed, 
and other factors, and are composed of ma- 
chinery noise and cavitation noise. The ma- 
chinery noise is produced by the engine and 
various auxiliaries while the cavitation noise 
is produced by the motion of the propeller and 
arises from the formation and collapse of 
cavities in the water. 

Surface Vessel Targets 

Figure 1 shows average spectra of an ideal- 
ized target with expected limits for high and 
low speeds. The spectrum of any individual 


result of machinery noise and occur in the 
middle sonic region. Figure 2, the measured 
spectrum of an aircraft-carrier, shows such 
a peak at approximately 1,100 C. Although the 




APPROX. LIMITING TREND FOR-/*^'-- 

VERY LOW SPEED (CAVITATION 
ABSENT OR UNIMPORTANT) 

^ L_ J_J 1 L I I 

1000 10,000 
FREQUENCY IN CPS 


HIGHLY VARIABLE UNCERTAIN 

Figure 1. Average frequency spectra of an ideal- 
ized target. 


amplitude of this peak is unusually large, peaks 
of large amplitude at one or more frequencies 
are not uncommon. 



1000 10,000 
FREQUENCY IN CPS 


Figure 2. Measured spectrum of an aircraft car- 
rier. 


target, however, shows peaks at various dis- 
crete frequencies. These peaks are usually a 

® See Division 6, Volumes 7 and 8. 


There are reasons for believing that ma- 
chinery noise and cavitation noise have differ- 
ent spectral shapes. The spectrum of machinery 


17 


18 


PHYSICAL FACTORS AFFECTING THE TARGET SIGNALS 


noise usually falls off at high frequencies at the 
rate of 12 db per octave or more, although 
strong single-frequency components, depend- 
ing on the details of the machinery, may exist. 
The slopes of observed cavitation noise spectra, 
in contrast, are always about — 6 db per octave. 
Thus the spectrum levels of cavitation noise 
are likely to be greater than the spectrum 
levels of machinery noise at high frequencies. 
The importance of cavitation noise probably 
increases with increasing speed; at very high 
speeds cavitation noise may be dominant over 
the entire 0.1- to 10-kc band. Machinery noise 
is more likely to be greater at low frequencies, 
and its relative importance probably increases 
with decreasing speed ; thus at very low speeds 
machinery noise may be dominant at all sonic 
frequencies. 

At the speeds at which surface targets 
usually move, cavitation noise is ordinarily 
greater than machinery noise at most fre- 
quencies above a few hundred cycles. However, 
the spectrum level of machinery noise may 
exceed the spectrum level of cavitation noise 
at frequencies where there are large machinery 
peaks. Such machinery peaks can occur any- 
where in the frequency band from 0.1 to 10 kc 
but are not likely to be dominant at frequencies 
above 2,000 c. 

Identification of targets by listening appears 
possible only through recognition of machinery 
sounds or through recognition of characteris- 
tic propeller-beat modulations. Since ma- 
chinery sounds cannot usually be heard at high 
frequencies, targets are probably more readily 
identifiable by sonic than by supersonic listen- 
ing. 

Experienced listeners, under favorable con- 
ditions, can distinguish between freighters, de- 
stroyers, battleships, and PT boats. Small boats 
can usually be distinguished from large boats. 
Some listeners can distinguish between empty 
and loaded freighters and between signals from 
targets at close range and at long range and 
can tell when a target changes course. Also, 
various other sounds can be recognized by 
sonic listening — torpedoes, depth charges, other 
submarines, airplanes, rain, fish noise, and 
noise from shoals and reefs. 


Submarine Targets 

In Figure 3 are plotted overall sound levels 
versus speed for large U. S. submarines at 
periscope depth. The dotted curve of the figure 
represents the expected radiated signal from 
the propellers, measured by a hydrophone 4 
feet from the propeller tip. It is apparent that 



I 2 3 4 5 7 10 

SPEED IN KNOTS 


Figure 3. Overall sound levels for large U. S. 
submarines at periscope depth. 


at speeds less than 4 knots the propellers con- 
tribute little to the composite energy radiated 
by the submarine. Though no reliable data exist, 
it is the opinion of most observers that at low 
speeds machinery sounds make up most of the 
radiated acoustic energy. This result is under- 
standable, since cavitation usually does not 
occur at speeds below 4 knots. 

One component of submarine target noise 
can be expected to show dependence on sub- 
marine depth. For a given speed, cavitation 
may be prominent when the submarine is near 
the surface and absent when the submarine is 
deeply submerged, since the speed at which 
cavitation sets in increases with increasing 
submarine depth. The other components of 
submarine target noise, such as noise from 
the motors and auxiliaries, should have little 
dependence on depth. 


NATURE OF TARGET NOISE 


19 


Torpedo Noise 

Figure 4 is a composite of measurements on 
several different types of torpedoes. At sonic 
frequencies, measurements show little correla- 
tion with either torpedo speed or torpedo type. 
Torpedo noise frequently shows marked fluc- 
tuations in time, with high peaks of short 



Figure 4. Noise measurements for several types 
of torpedoes. 


duration. The spectra also show many peaks 
at distinct frequencies, sometimes at frequen- 
cies as high as 10,000 c. The high-frequency 
sound from a torpedo shows minima directly 
ahead of and behind the torpedo, with increas- 
ing directivity at the higher frequencies. How- 
ever, at sonic frequencies this directivity is not 
very marked. 

Airplane Noise 

Because of the much greater sound velocity 
in water than in air, a sound ray from the air- 
plane striking the water at an angle of in- 
cidence greater than 13 degrees is totally re- 
flected. Rays striking the water at angles less 
than 13 degrees are almost completely reflected, 
but some energy does enter the water. These 
entering rays are refracted through an angle 


which increases with increasing angle of inci- 
dence and become horizontal for the 13-degree 
critical angle where total reflection begins. 
This results in a very rapid decrease of in- 
tensity in the water with increasing horizontal 
range from the plane. At the longer ranges, 
the intensity in the water decreases at the rate 
of 12 db per doubled horizontal range, which 
means that, in general, the airplane can be 
heard in a submerged hydrophone only in that 
part of the ocean immediately beneath the 
airplane. 

Figure 5 shows a typical airplane noise spec- 
trum measured af the surface beneath a plane 
in level flight. Considerable data obtained by 
various investigators show that low frequen- 



FREOUENCY IN CPS 


Figure 5. Airplane noise spectrum. 

cies predominate in airplane noise. Like tor- 
pedo noise, airplane noise often fluctuates 
markedly in time, with high peaks of short 
duration. Since the noise arises primarily from 
the propeller, harmonics of the propeller fun- 
damental are prominent. The sound radiated 
by the airplane is highly directional, even at 
100 c, with pronounced minima along the line 
of flight. 

Explosion Waves 

An explosion produces a shock wave consist- 
ing, at short ranges, of a very rapid, almost in- 
stantaneous increase in pressure followed by 
an exponential decrease with time. At ranges 
up to a few thousand yards, the peak pressure 
has a very simple dependence on the range (it 



20 


PHYSICAL FACTORS AFFECTING THE TARGET SIGNALS 


varies as the inverse 1.2 power of the range), 
and is almost independent of the weight of the 
depth charge. Thus, an estimate of the range of 
a depth charge explosion is possible from a 
measurement of the peak pressure of the pulse. 
An estimate of the bearing of the explosion, 
ahead or astern, to the port or starboard, and 
above or below, may be obtained by compar- 
ing the times of arrival of the pulse at 
appropriately situated pairs of hydrophones. 

33 TRANSMISSION LOSSES 

t 

Spreading and Attenuation 

Values of the transmission loss of sound in 
water at different frequencies are shown in 
Figure 6. Two effects are combined to give 
these curves. First, there is inverse square 



RANGE IN YARDS 

Figure 6. Transmission loss of sound in water. 

spreading, which weakens the sound by 6 db 
each time the distance over which the sound 
travels is doubled. Secondly, there is scatter- 
ing and absorption which weakens the sound 
by a fixed number of decibels each time a fixed 
distance such as 1,000 yards is added to the 
distance over which the sound travels. When 
transmission conditions are poor, as, for ex- 
ample, when temperature gradients are present 
near the surface, sound conditions are less well- 
understood; but, on the average, the attenua- 
tion coefficient for sonic frequencies under 
these conditions is about 2 db per kiloyard 
greater than in values given in Figure 6. 


Reflection 

Under certain conditions the sound intensity 
cannot be described as the result of inverse 
square spreading combined with absorption and 
scattering. For one thing, at ranges less than 

2.000 yards, sound of frequencies less than 

1.000 c may be markedly weakened by inter- 
ference with surface-reflected sound, if the 
sound source and listening hydrophone are 
fairly close to the surface. This image effect 
(or Lloyd mirror effect) is unimportant for a 
submarine listening to distant surface vessels, 
but it may be important for a submarine wish- 
ing to evade sonic detection by nearby surface 
craft. Also, inverse square spreading may not 
be the rule at all ranges in deep water posses- 



Figure 7. Sound output of a transport at 15 
knots. 



Figure 8. Weakening of sound by transmission 
losses. 


sing strong negative temperature gradients or 
in shallow water with such temperature gradi- 
ents over a soft mud bottom; for such oceano- 
graphic conditions, the downward bending of 
the sound rays may produce a shadow zone of 
very weak sound intensity at ranges beyond 500 
yards or 1,000 yards. In deep water, this sha- 
dow zone should not extend beyond a range of 





TARGET VERSUS BACKGROUND NOISE 


21 


several times the depth; sound reflected from 
the bottom should begin to come in at such 
long ranges ; and inverse square spreading and 
attenuation may be used to find the sound 
intensity as before. 

Figure 7 gives the spectrum of the sound 
output from a 15-knot transport; Figure 8 
shows the same sounds weakened by transmis- 
sion out to ranges of 1,000 yards, 8,000 yards, 
and 15,000 yards. The transmission loss val- 
ues are for the same good conditions portrayed 
in Figure 6. The assumed effect of absorption 
and scattering is evident in the progressive 
weakening of the high-frequency sounds. 

BACKGROUND NOISE 

When the target sound, modified by trans- 
mission, is received in the listening gear, it 
can be recognized if it can be distinguished 
from the other sounds heard in the loudspeaker 
or headphone. These unwanted sounds which 
tend to mask the sound being listened for are 
called background noise. 

Self-Noise 

The most important part of the background 
noise is the self-noise produced either by ma- 
chinery inside the ship or by the motion of 
water around the ship and listening hydro- 
phone. While the causes of self-noise in sonic 
gear have not been thoroughly investigated, the 
evidence indicates that most of the self-noise in 
JP-1 gear does not come from the submarine 
propellers. Measured self-noise levels in JP-1 
gear, installed in new-construction submarines, 
give the average values shown by the upper 
dashed curve in Figure 9. For individual sub- 
marines, the individual values vary by 10 db 
or more above and below the average curve. 
The self-noise is observed to increase with 
speed; at 4 knots, 6 knots, and 8 knots, the 
levels are on the average about 3 db, 10 db, and 
20 db respectively above the levels at 2 knots. 

System and Ambient Noise 

If a submarine stops and turns off all its 
noisy auxiliaries, the only background noise 
present is electric or system noise which, at 
sonic frequencies, is unimportant for well- 


designed gear, and the ambient noise which is 
present in the sea independently of the sub- 
marine. Ambient noise in the deep ocean arises 
primarily from agitation of the sea surface, 
and increases with wind force or sea state. 
Noise levels from this source received by JP-1 
gear in a number 2 sea state are shown by the 
lower dashed curve in Figure 9. Ambient noise, 



Figure 9. Background noise and target sounds in 
JP-1 gear. 


like cavitation noise, decreases about 6 db per 
octave when measured with a nondirectional 
hydrophone. The increased slope above 1,000 c, 
evident in Figure 9, results from the directiv- 
ity properties of the JP-1 gear. 

3 5 TARGET VERSUS BACKGROUND 
NOISE 

Also shown in Figure 9 are the target spectra 
at 8,000 yards and 15,000 yards taken from 
Figure 8. The target sound can be recognized 
at any frequency if, at that frequency, it is 
about as strong as the background noise. Thus 
screw sounds from the 15-knot transport can 
be heard at about 15,000 yards if self-noise is 
eliminated, whereas if the self-noise has the 
average value noted for JP-1 gear, these sounds 
can only just be heard at 8,000 yards. The 
recognition frequency is the frequency at which 
the target first becomes audible when the range 
is closed to maximum range. The recognition 
frequency is about 100 c for the lower pair of 
curves; however, listening at 1,000 c to 2,000 c 


22 


PHYSICAL FACTORS AFFECTING THE TARGET SIGNALS 


is only slightly less effective. When self-noise 
is important, the upper curves show that the 
recognition frequency for the cavitation sounds 
of this 15-knot transport is about 4,000 c. 

Consideration of the machinery peaks 
changes somewhat the conclusions based on 
cavitation sounds alone. When self-noise is 
negligible, the machinery peaks of Figure 7 
can be heard at about 20,000 yards. With all 
targets, machinery peaks of the height indi- 
cated can be heard at greater ranges than the 
cavitation sounds when self-noise can be neg- 


lected, though the range on the machinery 
peaks is rarely more than twice the range on 
screw sounds. When self-noise is high, how- 
ever, machinery peaks of the height indicated 
in Figure 7 do not extend the range much, if at 
all. (See Figure 9 for an illustration of this 
point.) Sometimes peaks much higher than 
those of Figure 7 are present in target signals. 
High, easily audible peaks most often occur at 
1,000 c or less; thus, to take full advantage of 
machinery peaks, background noise on sub- 
marines must be reduced at low frequencies. 



Chapter 4 



EXPERIMENTAL LISTENING SYSTEMS 




^2 the electrically steered 

SONIC SYSTEM 
Hydrophones and Circuits 

A line array of six BTL No. 5 type hydro- 
phones was mounted on 6-inch centers on each 
side of the hull of the Elcobel at a location 
representing the best compromise between 
maximum depth, distance from the propellers, 
and angle of incidence. The angle of incidence 


Depth is important because of the loss due to 
destructive interference between the direct 
sound waves and those reflected from the sur- 
face. The latter have more effect when the 
sound must travel close to the surface to reach 
the hydrophone. Distance from the propellers 
involves a loss of 6 db per double distance due 
to the divergence of the sound energy, plus 
whatever shielding can be obtained by inter- 
posing substantial-portions of the hull between 


INTRODUCTION 

A n investigation by the Bell Telephone 
^ Laboratories, Inc. [BTL] of underwater 
listening systems for use on small patrol craft 
was carried through the stages of preliminary 
survey, design, and construction of models, and 
installation on a small boat, the Elcobel. A 
description of the equipment and the tests made 
under controlled conditions is presented, since 
the basic principles and information gained 
are directly applicable to the design of detec- 
tion systems for use on both submarines and 
surface craft. 

This chapter describes four types of experi- 
mental systems which were designed and con- 
structed for this service as a result of prelimi- 
nary investigations and gives data on those 
characteristics of each system which deter- 
mine its suitability as a detection device. 

The following chapter discusses the results 
of field operating tests which were made in 
conjunction with Columbia University Division 
of War Research at the U. S. Navy Under- 
water Sound Laboratory, New London, Conn. 
[CUDWR-NLL]. These results are compared 
with the measured characteristics, and certain 
conclusions are drawn as to the value of the 
various systems and their components under 
operating conditions. These conclusions are 
summarized in a table of ratings which draws 
on all the information developed in this investi- 
gation, to mark each system according to its 
performance in each category. 


is the angle in a vertical plane between the face 
of the hydrophone and ' a sound ray from a 
source at the same depth as the hydrophone. A 
hydrophone mounted face down would have a 
zero angle of incidence. 


Figure 1. 5B hydrophone assembly mounted in 
the hull of the Elcobel. 


24 


EXPERIMENTAL LISTENING SYSTEMS 



the propellers and the hydrophones. The angle 
of incidence is again a question of the sound 
wave traveling along a reflecting surface, in 
this case the hull. 


The location selected is shown in Figure 1. 
This view shows the hydrophones extending 
from the hull and arranged for mounting a pro- 
tective cover. The arrays were 28 inches below 
the water line, 47 feet from the propellers at an 
angle of 40 degrees with the vertical. 

Hydrophones. The No. 5-type hydrophone,^ 
developed for use on the Elcobel, was designed 
to meet certain requirements derived from the 
preliminary survey. It was found that the lis- 
tening-range limitation of existing hydro- 
phones of small size was their noise threshold. 
The No. 5 type was designed for a threshold in 
the order of — 75 db at 5 kc ( — 1 db vs 0.0002 
dyne per sq cm per cycle). Its directivity pat- 
tern is very broad but this can be controlled 
by using sufficient units in an array. 

The starboard array consisted of six 5A 
hydrophones, one of which is shown in cross 
section in Figure 2. To provide the maximum 
signal-to-noise ratio, a two-stage coupling am- 
plifier is included in the 5A hydrophone hous- 
ing. The port array consisted of six 5B hydro- 
phones. These differ from the 5A in that a 
transformer is used in place of an amplifier to 
provide a low-impedance output. The purpose 
of using two kinds of coupling was to deter- 
mine the effect on the signal-to-noise ratio of 
running low-level leads from the hydrophone 
to the electric equipment. The use of an ampli- 
fier close to the crystals, although ideal from 
the standpoint of threshold noise, complicates 
the wiring and maintenance. It was thought 
that transformer coupling might be preferred, 
even if it meant extra shielding. 

Circuits for Array. A block schematic dia- 
gram showing the elements of the electrically 
steered sonic system is given in Figure 3. This 
includes a switch which introduces variable 
delay between the third hydrophone on one side 
of the boat and the corresponding one on the 
other side, effectively making a two-unit array 
which may be steered under the boat. The spac- 
ing between these two hydrophones is approxi- 
mately 30 inches, or the same as the fore and 
aft spacing of the six units for which the delay 
networks were primarily designed. 

An array or line of hydrophones operates on 
the phase-cancellation principle. For instance, 
two hydrophones can be spaced to that when a 
plane sound wave of a given frequency ap- 


THE ELECTRICALLY STEERED SONIC SYSTEM 


25 



26 


EXPERIMENTAL LISTENING SYSTEMS 


proaches along the line joining them, it reaches 
the second one at a maximum rarefaction of 
the medium at the same time that it reaches 
the first one at a maximum condensation. The 
two electric outputs are then 180 degrees out 
of phase and cancel completely where they are 
joined together. At any other angle of ap- 
proach, the effect is less. If an electric network 
is inserted between the two outputs so that the 
first is delayed sufficiently to bring it into phase 


spacing between hydrophones, / is the fre- 
quency, c is the velocity of propagation in the 
medium, and 0 is the angle with respect to the 
direction normal to the line. Near normal inci- 
dence the magnitude of phase shift is almost 
linear with respect to 6, but as the angle 0 ap- 
proaches 90 degrees a small phase shift cor- 
responds to a much larger change in 0. Changes 
in propagation velocity also have a greater 
effect on the accuracy as the angle approaches 



Figure 4. Schematic diagram of auxiliary phasing amplifier. 


with the second, a maximum results instead of 
a null. Therefore, by using a variable delay net- 
work the beam pattern of the array can be 
steered over any desired angle. 

The amount of phase shift a between ad- 
jacent hydrophones that is required to steer 
the directivity pattern of a line array over an 
angle of 90 degrees depends upon the factor 
27rlf/c sin 0, where I is the center-to-center 


90 degrees. In the circuit to be described, the 
phase delay is varied by the resistance poten- 
tiometer method. The five potentiometers are 
on a common shaft that is driven through heli- 
cal gearing with a 4-to-l reduction by a crank 
shown on the front of the panel. 

The additional delay required to bring the 
output of one-half of the array into phase with 
the output of the other half for use with a 





THE ELECTRICALLY STEERED SONIC SYSTEM 


27 


phase-operated left and right indicator system 
is provided by an auxiliary panel (coded 
D-165061) whose schematic diagram is shown 
in Figure 4. The operation of the phase-actu- 
ated locator [PAL] and right-left indicator 
[RLI] systems is discussed in Chapter 6. 

In an array of hydrophones the loss varia- 
tions of the several networks in tandem are 
added together, and when the variation from 
one end to the other is integrated over the fre- 
quency range from 1 to 10 kc for all the hydro- 
phones of the array, no inequality is noticeable 
to the ear. 



Figure 5. Front panel assembly of coupling and 
phasing amplifier. 

Figure 5 is a photograph of the front panel 
assembly of the coupling and phasing amplifier. 
The four pointers shown in the photograph 
turn with a scissor-like motion so that with no 
delay the starboard pair coincide and the port 
pair coincide. As the delay is increased, one 
pointer on each side moves up and one moves 
down simultaneously. 

The handwheel under the dial varies all five 
networks simultaneously. When the keys on 
the switching panel are thrown to fore and 


starboard, the upper right-hand quadrant is 
illuminated. The arm covering this quadrant 
moves from the vertical to the horizontal posi- 
tion as the handwheel is turned clockwise, the 
delay changes from 100 per cent to 0 per cent, 
and the lobe of maximum response is swept 
from dead ahead to abeam. To go past abeam, 
the key is thrown to aft. This illuminates the 
after quadrant, the hydrophones are reversed 
with respect to the delay and, when the hand- 
wheel is turned counterclockwise, the delay 
changes from 0 per cent to 100 per cent and 
the lobe of maximum response moves aft. The 
four indicating arms are arranged so that their 
anplar displacement with respect to either 
axis is always the same. Therefore, when 
switching from one quadrant to another, the 
angular change can be made as small or as 
large as desired. This is useful in making a 
rapid survey of the four quadrants to insure 
that the loudest response is within the quadrant 
under examination. It is also useful in deter- 
mining whether a target is dead ahead, since 
by throwing the key between port and star- 
board it can be observed when the target 
crosses the bow. 

Measured Characteristics 

Frequency Response. The frequency response 
of the electrically steered sonic system is shown 
in Figure 6 in terms of the power into a 600- 



Figure 6. Frequency response of electrically 
steered sonic listening system. 

ohm load at the output terminals of the phasing 
amplifier versus the sound pressure in the water 
at the location of the hydrophones. The scale is 
cross-referenced in terms of 0.0002 and 1 dyne 




28 


EXPERIMENTAL LISTENING SYSTEMS 


per sq cm per cycle. The latter is the present 
standard and is included on all drawings in- 
volving pressure-spectrum level. The response 
below 1 kc is not shown, since it was found by 
means of the variable filters that most of the 
noise caused by local sources close to the hull 
and by vibration which passed through the 
isolating mounts could be eliminated by includ- 
ing a 1-kc high-pass filter in the circuit. Al- 
though there are some differences in the re- 
sponse of the two types of hydrophone, they do 
not affect the impression gained by listening 
over the range from 1 to 10 kc. The same may 
be said of the effect of the hull on frequency 
response. There was found to be little differ- 
ence in the response of the starboard array 
mounted in the hull and that of the same array 
mounted in a free sound field, except at high 
frequencies (above 8 kc) where the free-field 
response was about 10 db higher. The uni- 
formity of the hydrophones as shown by indi- 
vidual free-field response curves is an impor- 
tant factor in maintaining the directive 
properties of the arrays. Any uncontrolled 
resonances within the listening frequency band 
caused by structural deficiencies would result 
in phase shifts between units which would 
upset the artificial phase relations established 
by the networks. Indicator circuits such as the 
PAL are particularly sensitive to such uncon- 
trolled phase changes. 

Directivity Patterns. The change in the 
response of a hydrophone or array of hydro- 
phones as it is rotated in the sound field pro- 
duced by a fixed source some distance away is 
commonly measured at single frequencies. Di- 
rectivity patterns obtained in this way are 
used to confirm design computations and to 
check uniformity of product. They give a rather 
diffuse picture of what to expect when listen- 
ing with the device, since listening is done with 
a band of frequencies and the effect cannot be 
described by a single-frequency pattern. This 
is particularly true in the sonic range where 
the listening bandwidth is much wider in 
octaves than a supersonic listening band of the 
same width in kilocycles. 

The patterns presented in this report were 
measiired with a band of thermal noise as the 
source. This is not completely descriptive 
either, as the ear may exercise its sense of pitch 


to discriminate, at least between high and low 
frequencies. It is well known that a listener 
may concentrate, just as if he were inserting a 
high-pass filter, on the higher frequencies 
whose sharper beam enables him to get a more 
accurate bearing, after he has used the overall 
loudness to locate the sound source. For this 
reason some of the patterns are shown for two 
noise bands, one for the high frequencies and 
one for the overall band, to give some idea of 
the type of pattern available for bearing deter- 
mination by listening alone. 

The directivity patterns of the electrically 
steered sonic array are given in Figure 7. The 


330* 0 30” 



signal-thermal NOISE DECREASING 6 DB PER OCTAVE 
LISTENING BANDS — l-IOKC 5-10 KC 

Figure 7. Directivity patterns of electrically 
steered sonic system; signal from fixed sources 
during electric rotation. 

chart is in two halves, one for a signal coming 
from the starboard beam and the other for a 
signal coming from the port bow. Several char- 
acteristics of hull-mounted arrays are illus- 
trated by these patterns. One is the blunting 
of the lobe as it moves away from abeam. This 
is a characteristic of electric steering and is due 
to the cumulative effect of departures from an 


CONpefNfiAL 


THE ELECTRICALLY STEERED SONIC SYSTEM 


29 


exact linear relation between phase shift and 
frequency. The lobe at 330 degrees would actu- 
ally be broader if it were not for the effect of 
the boat hull which tends to reduce the response 
near the bow. Another characteristic is the 
sharpening of the main lobe by restricting the 
bandwidth to the upper end of the spectrum. 
This makes for greater bearing accuracy but 
has the disadvantage of increasing the ampli- 
tude of the minor lobes. This is a character- 
istic of all arrays. Reducing the wavelength re- 
sults in narrower lobes which, however, occur 
more frequently, the pattern eventually repeat- 
ing itself at decreasing angular separations. 

The effect of the boat hull has been obtained 
by measurements on the Elcobel, as shown in 
Figure 8. The hull provides shielding between 



6 DB PER OCTAVE 

Figure 8. Effect of hull on electrically steered 
sonic system. 

port and starboard in the order of 20 db or 
more for the sonic range. This is one of the 
advantages that comes with hull mounting and 
plays a part in deciding the location of sound 
gear on any type of boat. The loss due to graz- 
ing incidence at the fore and aft positions is a 


serious disadvantage. It not only reduces the 
sensitivity at those positions but causes a loss 
in signal-to-noise ratio because the directivity 
index does not change appreciably with angle 
of train. 

The directivity index is a ratio of the volume 
enclosed by the directivity pattern, treated as a 
solid of revolution about the appropriate axis, 
to that of the sphere with radius of maximum 



6 DB PER OCTAVE 


Figure 9. Directivity pattern of electrically 
steered sonic system; ambient noise at two fixed 
electric positions. 

response. It is a measure of the discrimina- 
tion against ambient noise. The pattern used 
for computing the directivity index is shown 
in Figure 9. It was obtained by leaving the 
array aimed at the original position of the sig- 
nal and then moving the signal around the boat 
to simulate random noise. As shown in Figure 
7, the loss incurred at the bow and stern may 
cause a side lobe to exceed the main lobe in in- 
tensity. This may lead to ambiguity, as by 
steering near the bow on a signal which is 
actually in the aft quadrant. For this reason 
it is necessary to scan all four quadrants to 





30 


EXPERIMENTAL LISTENING SYSTEMS 


locate all possible sources of interference 
before obtaining a bearing. For antisubmarine 
work this is not a serious disadvantage 

The combined effects of hull mounting and 
electric steering are shown in Figure 10. This 
presents a pattern for each of the four com- 



6-UNIT ARRAY OF 5A HYDROPHONES 


HULL MOUNTED ELECTRICALLY STEERED 

FREE FIELD ELECTRICALLY STEERED 

HULL MOUNTED MECHANICALLY STEERED* 

FREE FIELD MECHANICALLY STEERED 

tsiGNAL MOVED AROUND HULL) 

SIGNAL l-IOKC NOISE BAND 
DECREASING 6 DB PER OCTAVE 

Figure 10. Directivity pattern showing combined 
effect of hull mounting and electric steering. 

binations of hull versus free field and mechani- 
cal versus electrical steering. The basic pattern 
is that for a mechanically steered array in a 
free field. When the array is mounted in a hull 
but still steered mechanically, the pattern shows 
a slight broadening of the main lobe and the 
characteristic loss fore and aft. If the array is 
left in a free field and steered electrically, 
there is a slight narrowing in the main lobe 
but, principally, a broadening of the side lobes 
at the end points. The combination of hull 
mounting and electric steering gives a rather 
broad pattern with reduced side lobes. The 
effect of the hull may be likened to a tapered 
array, the taper being introduced by the suc- 
cessive losses caused by grazing incidence along 
the hull between hydrophones. 

The patterns obtained by steering the two- 
unit array under the boat would be rather poor, 
characteristic of a two-unit array with wide 
spacing. However such an array used with the 
PAL circuit and a broad frequency band pro- 
vides a good indication of bearing in the ab- 
sence of secondary targets. 

Another directivity pattern of interest is that 
showing the effect of projecting the hydro- 


phones from the hull. Computations had indi- 
cated a considerable improvement to be ex- 
pected by moving the hydrophones out from the 
hull. This improvement is due to the fact that 
the direct and refiected waves at the boundary 
of the water and lower impedance medium (the 
boat hull) are 180 degrees out of phase. As the 
hydrophone is moved away from the boundary, 
the phase difference becomes less, and instead 
of a loss there may be a gain in sound pressure. 

Self -Noise. The spectra of self -noise with 
the boat underway for three speeds are given 
in Figure 11. These show the self -noise varying 
inversely with frequency more rapidly than is 
the case with ambient noise, which indicates 
that the higher frequencies are more suitable 
for listening while underway. 



7 1000 2 3 5 7 10,000 2 


-4 

lij 

-j 

-14 

o 

>- 

o 

\ 

-24 

r 

c> 

o 

-34 

lU 

-44 

z 

>- 

o 

-54 

tf) 

> 

-64 

-74 

CD 

o 


Figure 11. Self-noise spectra of electrically 
steered system (No. 1 sea) in terms of equivalent 
sound pressure at hydrophones. 


Self-noise is a combination of noises from 
at least three sources. One of these is propeller 
cavitation. Another is engine vibration trans- 
mitted either directly through the hull or from 
the hull to the water and thence to the hydro- 
phone. The underwater engine exhaust may 
also contribute to this. Third source of self- 
noise is the action of the water against the 
hull and hydrophone. When the boat is under- 
way, this noise increases. The proper method 
of determining the effect of each source is to 
measure each independently. For instance, the 
effect of maintaining engine speed correspond- 
ing to a 4-knot water speed without motion 
through the water shows that at the upper end 
of the spectrum engine noise disappears into 


THE ELECTRICALLY STEERED SONIC SYSTEM 31 


the ambient but at the low frequencies its effect 
is important. 

Isolation of propeller noise is more difficult 
to accomplish. Measurements made with hydro- 
phones under the Elcobel trained on the pro- 
pellers indicate that the slope of the spectrum 
is about 5 db per decade. This is in good agree- 
ment with measurements of ship’s propellers 
reported elsewhere.^ It is also necessary to 
take into account the effect of hull shielding 
which discriminates against the high frequen- 
cies. 

To find the effect of motion through the 
water, tests were made with the boat in motion 
and with it tied to a dock while the engines 
and propellers were maintained at a speed cor- 
responding to 9 knots. Such tests are more 
reliable as to slope than magnitude, since the 
level of propeller cavitation undoubtedly in- 
creases when the boat is restrained. They defi- 
nitely indicate, however, that motion through 
the water is a heavy contributor to the low 
end of the spectrum. 

With the isolation of propeller noise and the 
noise due to motion through the water, it is 
possible to break down the total noise spectra 
into their component noise sources. The result 
is given in Figure 12 for two boat speeds. These 
analyses are based on two requirements for the 
component spectra: one, that they conform in 
slope to the considerations discussed above and, 
two, that they add up on a power basis to the 
total noise curve as measured. These curves 
which, for lack of precise information, have 
been drawn as straight lines indicate that for 
the sonic arrays the controlling noise is due to 
motion through the water. The upper end of 
the frequency band departs least from the 
ambient noise for the existing sea conditions, 
and at low speed such departure is due almost 
entirely to noise from the propellers. 

A streamlined cover over the six-unit array 
was tried. Over the listening band up to 10 kc, 
the cover does not offer much improvement. At 
low speeds, there is a small decrease in the 
self-noise, but at higher speeds it increases. 
Confirmation of this was obtained in connec- 
tion with self-noise measurements on the elec- 
trically steered supersonic arrays discussed in 
the next section. These arrays were located 
directly forward of the six-unit arrays. It was 


observed that the self-noise picked up by the 
supersonic arrays when they were steered aft 
was considerably higher when the cover was 
on the six-unit arrays. 

Internal Noise. Internal noise is defined as 
the noise introduced in the listening system by 
the electric equipment. Basically, it is the 
thermal noise in the resistive component of the 
impedance at the lowest-level point of the cir- 



Figure 12. Analyses of self-noise of electrically 
steered sonic system; No. 1 sea. 


cuit. Other noise sources may predominate, 
however, particularly noise in the amplifier 
tube immediately following the low-level point. 
This is commonly referred back to the input 
of the tube as the series grid resistance whose 
thermal noise would produce an equivalent level 
at the output. In an array the threshold is 
lower, since the active area of sound pickup is 
increased without changing the effective re- 
sistance at the low-level point. Internal noise 
spectra for hydrophone systems in this report 




I 


32 


EXPERIMENTAL LISTENING SYSTEMS 


are given in terms of the sound pressure in 
the water which would produce the same volt- 
age as the observed circuit noise. The spectrum 
for the internal noise of the electrically steered 
sonic system is shown in Figure 13. 


\ 









- >> 1 

PORT 1 

[5B) 




STARBC 

)ARD (! 


sX 









y 

, 


-54 y 


2000 5000 10,000 20,000 o 

FREQUENCY IN CPS 


Figure 13. Internal noise spectra of electrically 
steered sonic system in terms of equivalent sound 
pressure at hydrophones. 


THE ELECTRICALLY STEERED 
SUPERSONIC SYSTEM 

The need for investigating supersonic listen- 
ing systems is clearly indicated by the fre- 
quency spectra of self-noise with the boat 
underway (Figure 12) and also the rather broad 
directivity patterns associated with sonic fre- 
quencies. It is true that any beam width can 
be obtained for any frequency range by using 
a sufficient number of hydrophones and spac- 
ing them properly. At the low sonic frequen- 
cies, sharp beam patterns with low side lobes 
can be obtained only by using an array whose 
dimensions approximate the length of the boat. 
The characteristics of a 3-foot, sonic array 
showed that the region above 5 kc is better 
suited to bearing determination by listening 
than the lower frequencies. 

Although supersonic frequencies are attrac- 
tive both from the standpoint of the reduction 
of self-noise and of the increase of directivity 
obtainable with small-sized units, there are sev- 
eral factors to keep in mind. The attenuation of 
sound energy in water increases with fre- 
quency very rapidly beyond 20 kc. The differ- 
ence between 5 kc and 20 kc is small compared 
with the reduction in self-noise and the im- 
provement in directivity obtainable when size 
is a determining factor. The results obtainable 
by listening above 30 kc were not investigated 
in this study. The type of signal being detected 


is another factor. Ship propellers turning 
above cavitational speeds are rich in super- 
sonics. Below cavitational speed, which may be 
as high as 6 knots for a submarine at 300 feet, 
supersonic frequencies are reduced some 40 db. 
The ability of a submarine employing evasive 
tactics to maneuver at reasonable depths with- 
out creating high-frequency noise makes its 
detection by listening from a small boat under- 
way very difficult. 

Hydrophones and Circuits 

Hydrophones. The supersonic arrays con- 
sisted of two Type 7A assemblies of Rochelle 
salt crystal hydrophones. The details of the 
construction are shown in Figure 14. As the 


HOUSING FOR REPEATING COIL 
-CAST HYDROSTATIC BRONZE COIL HOUSING 
TWO- CONDUCTOR SHIELDED CORDS- 


45 » "X" CUT 
ROCHELLE SALT 
CRYSTAL .175 
INCH X .700 INCH 
X.575 INCH 


-RUBBER 
GASKET 
GLAND RING 



-GLASS BEAD 
CONNECTOR 

-RUBBER GASKET 
-"NADAC" HOUSING 


.015-INCH BERYLLIUM 
COPPER DIAPHRAGM 


-.040-INCH CERAMIC 
-iV-lNCH CERAMIC 
-"NADAC" RESIN ATOR 
- KORITE 


Figure 14. Cross section of 7A hydrophone elec- 
trically steered supersonic system. 


result of experience with the 5A-type hydro- 
phones, no protective cover was used with 
these arrays. However, an attempt was made 
to reduce the hull effect by framing the array 



THE ELECTRICALLY STEU^ED SUPERSONIC SYSTEM 


33 


with a i/4-inch brass plate which extended for 
several wavelengths fore and aft of the unit. 
The use of metal instead of wood in the hull 
should considerably reduce the attenuation of 
the sound wave at grazing incidence. 

Electric System, The amplifier and phasing 
system used with the supersonic arrays is simi- 
lar to the sonic system shown in Figure 3. In 
this case, no attempt was made to steer the 
array under the boat on account of the wide 
spacing. The leads from each of the nine ele- 
ments were brought to their own coupling am- 
plifier from which they passed through succes- 
sive phase networks. A block of networks is 
used to bring the two halves of . the arrays 
(omitting the ninth unit) into the PAL circuit 
as before. 

The listening system makes use of the output 
from all nine units and a heterodyne system is 
used to bring the frequency band from the 
supersonic range down to the audible range. A 
1-kc high-pass filter is included to prevent 1-f 
noise from overloading the heterodyne system. 

Measured Characteristics 

FveQueTicy Response. The overall frequency 
response of the supersonic system is shown in 
Figure 15. This response is fairly uniform over 



Figure 15. Frequency response of electrically 
steered supersonic system measured at output of 
phasing amplifier. 


a broad band of frequencies. However, there 
are limitations to its usefulness outside a cer- 
tain range. At the low frequencies, the direc- 
tivity becomes poor. At the upper end of the 
band, there is a resonance around 26 kc be- 
tween the crystal capacity and the transformer. 
As the location of this resonance differed 
among units, the listening range was restricted 
to the band from 13 to 23 kc. 


Directivity Patterns. The directivity pat- 
terns obtained with a band of thermal noise de- 
creasing 6 db per octave are given in Figure 
16. The patterns are for two angles of incidence 
as the array is steered electrically through the 
signal. Comparison with similar patterns in 



6 OB PER OCTAVE 

Figure 16. Directivity pattern of electrically 
steered supersonic system ; signal from fixed 
sources during electric rotation. 

Figure 7 shows a considerable improvement in 
the beam width and particularly in side lobe 
reduction due to the greater number of ele- 
ments. There is no need to divide the band into 
two parts because the octave range is small and 
also because the 10-kc band selected by the 
modulator is reduced to 5 kc for listening. 

The directivity index versus frequency is 
shown in Figure 17. The effect of the hull on 
the sound level reaching the hydrophone is 
shown in Figure 18. The per cent of the cir- 
cumference over which the effect is small has 
been increased as compared with that obtained 
with the sonic arrays (Figure 8) . This has been 
enhanced by the brass plate surrounding the 
7A hydrophone. It effectively substitutes a 
high-impedance for a low-impedance reflector 




34 


EXPERIMENTAL LISTENING SYSTEMS 


in the vicinity of the units, thereby avoiding 
phase cancellation along the reflector. The effect 
is now one of summation which varies some- 
what with the angle of train as shown by the 



Figure 17. Directivity index of 7 A hydrophone. 

irregularities in the pattern. This method is 
effective only at frequencies for which the 
wavelength is small, since the reflector must 
extend beyond the hydrophones for several 
wavelengths to be of any use. 



SIGNAL-THERMAL NOISE 13*23 KC BAND DECREASING 
6 DB PER OCTAVE 

Figure 18. Eifect of hull on listening with the 
7A hydrophones. 

Self -Noise and Internal Noise. The noise pro- 
duced in the supersonic arrays steered abeam 
when the boat is underway was measured at 4, 
6, and 9 knots. At the low speeds the level of 


self-noise is so close to ambient that an analysis 
is not justified. This leaves only the 9-knot 
speed to be broken down into its components, as 
shown in Figure 19. The noise caused by mo- 
tion through the water plays a minor role at 
supersonic frequencies. The main contribution 
is from the propellers except at the lower end 



Figure 19. Analysis of self-noise at 9 knots of 
electrically steered supersonic system. 


of the spectrum. The propeller noise spectrum 
is a continuation of that used for the sonic 
range, and, as in the sonic range, it depends 
primarily on the shielding effect of the hull. 

4^ THE MECHANICALLY STEERED 
SONIC SYSTEM 

The principal disadvantage of the electrically 
steered sonic arrays tested is the relatively 
poor directivity which, because of the effect of 
the boat hull, leaves a sector of uncertain bear- 
ing accuracy about 20 degrees either side of the 
bow and stern. Electric steering also presents 
difficulties in design and construction which 
make it inherently expensive. These disadvan- 
tages can be avoided to a large extent by using 
a similar array suspended below the boat and 
steered mechanically. The greater depth at 
which such an array would operate is a further 
advantage. The effect of the hull is not com- 
pletely eliminated and for extreme accuracy 
must be taken into account. 


THE MECHANICALLY STEERED SONIC SYSTEM 


35 


Hydrophone and Circuits 


A hydrophone, coded 9AA, was made up us- 
ing six inertia-type, permanent-magnet units 



spaced 5 inches apart. This unit provided a 
directivity pattern with a considerable reduc- 


tion in the rear response. A mechanism for 
raising and lowering the array and for train- 
ing it on the target was installed. 

The 9AA hydrophone is shown in cross sec- 
tion in Figure 20. The six units were mounted 
on a frame which held them in a water-filled 
chamber making up the front section of a wing- 
shaped stainless-steel body. The rear section 
was air-filled to serve as a baffle for reducing 
the rear response. The directivity pattern of 
each unit is useful in this application to avoid 
the effects of reflections from the rear wall of 
the front chamber as well as to reduce the rear 
response of the assembly. The end pieces of the 
body were also air-filled to reduce the side 
response. A photograph of the hydrophone in 
place under the starboard side of the Elcobel 
is shown in Figure 21. 



Figure 21. 9AA hydrophone mounted on star- 
board hull of Elcobel. 


The principle of operation of the inertia unit 
is simple. An armature is suspended over the 
pole pieces of an electro magnet by a flat spring 
member. The magnet is mounted on the inside 
of a steel sphere. Motion of the sphere is com- 
municated to the magnet which causes the 
armature to move. Relative motion between 
the armature and the magnet alters the mag- 
netic field and induces a current in the winding. 
The natural period of the mass of the armature 
and the stiffness of the spring member is about 
1,000 c. The steel shell and the magnet resonate 
near 10 kc. These resonances can be shifted and 







36 


EXPERIMENTAL LISTENING SYSTEMS 


broadened by the use of mechanical damping 
in the form of rubber mounted in the air gap. 

A block diagram of the electronic equipment 
used with the mechanically steered sonic array 
or 9AA hydrophone is shown in Figure 22. 


to match effectively each unit to the input cir- 
cuit at the low end of the frequency range in 
order to obtain a more uniform frequency 
response. The frequency responses of the indi- 
vidual units showed uniformity not only over 



9AA HYDROPHONE 


Figure 22. Block diagram of mechanically steered sonic system. 


Measured Characteristics 

Frequency Response. The frequency response 
of the 9AA hydrophone is shown in Figure 23 
for 0-degree and 180-degree incidence of the 
sound wave. It will be seen that the voltage 



Figure 23. Frequency response of mechanically 
steered six-unit sonic array. 


developed for a sound pressure of 0.0002 dyne 
per sq cm is relatively small. This is primarily 
due to the low impedance of the six units in 
parallel. The parallel connection was used so as 


the frequency range but among the units, which 
is an advantage for the use of phase-operated 
left and right indicator systems. 

Directivity Patterns. The directivity pat- 
terns for the 9AA hydrophone were measured 
in two ways, at a lake test station using a band 
of noise with a slope characteristic of ship 
sounds and on the Elcohel using a band of 
noise with uniform response over the spectrum. 
The uniform frequency band places more 
emphasis on the high frequencies, resulting in 
a narrower beam width as shown by the pat- 
terns in Figure 24. The directivity index for the 
9AA hydrophone is plotted as a function of 
the frequency in Figure 25. 

Self-Noise. The self-noise of the 9AA hydro- 
phone is shown in Figure 26. The directivity 
patterns of self -noise at two speeds, shown in 
Figure 27, differ in appearance from patterns 
of the hull-mounted systems. The patterns 
bulge out toward the rear by the amount of 
shielding provided by the baffle and are essen- 
tially an indication of how this shielding varies 
with angle of train. It was observed that at 
high speed, the noise level in the water is higher 




THE MECHANICALLY STEERED SUPERSONIC SYSTEM 


37 


for the hull-mounted arrays abeam and lower 
fore and aft. This is due to the proximity of 
the bow wave and to the hull shielding, and ties 
in with the conclusion reached earlier that 
motion through the water is the controlling 
noise source for the hull-mounted sonic arrays. 


This part of the band is, therefore, seriously 
restricted in the level of signal which can be 
distinguished above the internal noise. The 
noise level at the lower end of the band is not 
a limiting factor because self-noise and ambient 
noise are more likely to be controlling. 




90 * 


Figure 24. Directivity patterns of mechanically steered sonic system measured using different signals. 


Internal Noise. The threshold noise for the 
9AA hydrophone, as compared to that for the 
hull-mounted sonic arrays, indicates a serious 



'000 2 3 5 7 10,000 

FREQUENCY IN CPS 

Figure 25. Directivity index of 9AA hydrophone. 


deficiency of the 9AA in this respect. At the 
upper end of the frequency band, the noise 
threshold of the 9AA is 20 to 30 db higher. 


THE MECHANICALLY STEERED 
SUPERSONIC SYSTEM 

The same advantages apply to the use of a 
mechanically steered supersonic system as to a 
sonic system, namely, the elimination of the 
boat-hull interference as it affects the azimuth 
coverage and the removal of the complication , 
of electric steering. Since a supersonic fre- 
quency band provides good directivity with a 
hydrophone of small size, a unit with an active 
face smaller than that of the 7A array operat- 
ing at a somewhat higher-frequency range was 
designed. This unit was later developed into the 
No. 6 series of hydrophones and projectors. 

Hydrophone, Dome, and Circuit 

As originally designed for this system, the 
No. 6 type hydrophone discriminated against 




38 - 


EXPERIMENTAL LISTENING SYSTEMS 


signals coming from the rear of about 30 db. 
It was thought that this could be improved by 
using a baffle in a streamlined dome, which at 
the same time would eliminate cavitation and 
turbulence due to motion through the water 



NOISE FROM ENGINES 

Figure 26. Self-noise of 9AA hydrophone in- 
stallation. 


at moderate speeds. The 6C hydrophone which 
was finally employed in the dome is shown in 
cross section in Figure 28. 

A half-sized model of the QBF dome was 
built to include a castor oil filled baffle. The 
steel casting of the baffle was later covered 
with l^-inch cork rubber to improve its shield- 
ing. The stainless-steel acoustic window was 
not only half-sized but half the thickness of that 
used in the QBF dome, 0.010 inch instead of 
0.020 inch. Since this dome was completely 
submerged, a top section similar to the bottom 
section was added and provided with a fiange. 
A block diagram of the circuit used with the 
6C hydrophone is shown in Figure 29. 


Measured Characteristics 

Frequency Response. The 6C hydrophone has 
a broad frequency range extending from below 
10 kc to beyond 60 kc. The region from 20 to 
30 kc was selected for high efficiency and satis- 
factory beam width, but listening could be 
done above and below this range if desired. 
However, the effect of the dome on frequencies 
above 35 kc was not investigated and, there- 
fore, the operation of the phase sensitive PAL 
circuit was confined to the 20- to 30-kc band. 



Figure 27. Directivity patterns of self-noise of 
mechanically steered 9AA hydrophone. 


Directivity Patterns. The directivity patterns 
shown in Figure 30 illustrate the effect of the 
dome. The 57-degree beam width is not so 
narrow as that of the supersonic array but 
gives a fairly good indication of bearing with 
very little interference from side lobes. At 330 
degrees a side lobe appears which is entirely 
due to specular refiection from the acoustic 
window. The function of the baffle is twofold. 
Its primary purpose is to shield the hydrophone 
from the rear. It also provides absorption 
inside the dome chamber for multiple refiections 


/ 


\ 


APPROX 26^ IN 


THE THROUGH-THE-HULL SONIC LISTENING SYSTEM 


39 


which would seriously distort the directivity 
patterns. 



Figure 28. Cross section of 6C hydrophone; 
mechanically steered supersonic system. 


The effect on the directivity pattern of the 
cork-rubber cover placed over the back of the 
6C hydrophone is shown in Figure 31. This 
was measured without the dome and shows a 
considerable reduction in rear response which 
is valuable in reducing the effect of a nearby 
noise source such as the propellers. In apply- 
ing the cork rubber, thej sides of the hydro- 
phone appeared to be most important, and the 
greatest ' improvement was obtained by bring- 
ing the cork rubber as far forward as possible 
without shielding the crystal face. 

Self-Noise. The frequency spectra of self- 
noise measured with the hydrophone trained 
forward are shown in Figure 32. The steep 
slope of these spectra is caused by the fre- 
quency discrimination of the baffle. The differ- 
ence between the front and back response of 
the 6C hydrophone also contributes to the 
slope of the self-noise spectra, although its 
effects are somewhat confused by reflections 
within the dome. It is interesting to note that 
the absolute levels of self-noise are several 
decibels higher than those obtained with the 
hull-mounted 7A hydrophones (Figure 19). 
However, by using a hydrophone in the dome 
with a beam width comparable to that of the 
7A, the self-noise level is reduced. 


THE THROUGH-THE-HULL SONIC 
LISTENING SYSTEM 

Response Characteristics 

A production-type JP-1 through-the-hull sys- 
tem was installed on the Elcobel to serve as a 
basis for comparison with the several experi- 
mental systems already described. This system 
is a mechanically steered sonic system using a 
3-foot line hydrophone capable of responding to 
frequencies over the entire sonic and the lower 
supersonic regions. Included in the system was 
an amplifier whose frequency characteristic 
could be varied by means of a series of filters. 
A resonant circuit (bass boost) was also avail- 
able to raise the response in the region of 
100 c for special listening conditions. The fre- 
quency response of the amplifier is shown in 
Figure 33. 


40 


EXPERIMENTAL LISTENING SYSTEMS 


o 

o 


O 

O 

u> 


WESTON MODEL 301 
lOO-O-lOO 
MICROAMMETER 




D-169902 


PHASE 



ACTUATED 



LOCATOR 



6C HYDROPHONE 


BRUSH TYPE Al /wS 
HEADPHONES 


Figure 29, Block diagram of circuit for the 6C hydrophone. 


330 


300 


270 


240 



330“ 


A 210- ISO' |5o‘ 

SIGNAL : THERMAL NOISE 20*30 KC BAND DECREASING 
6 DB PER OCTAVE 


300 


270 


240 



120 


B 210“ 180“ 150“ 

signal: THERMAL NOISE 20-30KC BAND DECREASING 
6 DB PER OCTAVE 


Figure 30. Directivity patterns of 6C hydrophone in 25-inch dome. 


24,000 




THE THROUGH-THE-HULL SONIC LISTENING SYSTEM 


41 


Directivity Patterns 

The directivity patterns of the JP-1 hydro- 
phone were measured in two ways as in the 
case of the 9AA hydrophone. The results of 
these measurements are shown in Figure 34. 
The difference in patterns between the 1- to 
10-kc band and the 5- to 10-kc band is very 
much less than it was for the 9AA hydrophone 
(Figure 24). This is caused by the comparative 
lack of low-frequency response which would 
otherwise broaden the pattern when included. 
If it is considered necessary to use the lower 
frequencies to detect particular types of sig- 
nals, these may be emphasized by means of low- 
pass filters or the bass boost. 



Figure 31. Directivity pattern of 6C hydrophone 
with and without cork-rubber back. 


Self-Noise and Internal Noise 

The JP-1 hydrophone, due to its unshielded 
magnetic circuit, is very susceptible to electric 
interference produced by the engine ignition 
system. For this reason, self-noise underway 


is essentially nondirectional. At 6 knots the 
interference completely masks the propeller 
noise, even when the hydrophone is aimed 



Figure 32. Self-noise spectrum of 6C hydrophone 
in dome; No. V 2 sea. 


astern. At 9 knots only the rear shows the 
effect of the propellers above the noise from 
electric interference. 



Figure 33. Frequency response of amplifier for 
through-the-hull equipment. 


The internal noise threshold of the JP-1 
hydrophone is approximately 7 db better at the 
upper end of the sonic frequency range than 
that of the 9AA hydrophone. 


42 


EXPERIMENTAL LISTENING SYSTEMS 


330» 


30» 


330“ 


30“ 



60« 


90 « 


I20« 



60‘ 


90* 


I20« 


Figure 34. Directivity patterns of JP-1 hydrophone; measured using different signals. 




Chapter 5 


COMPARISON OF EXPERIMENTAL SYSTEMS 


I T IS NOW PROPOSED .to evaluate the character- 
istics of the four listening systems on the 
Elcohel and to evolve in the process a set of 
criteria by means of which they and listening 
systems in general may be compared.® These 
characteristics are, to a certain extent, inter- 
linked. The importance to be attached to each 
is a matter of judgment and depends on the 
environment in which they are to be used. The 
internal noise threshold, for instance, is unim- 
portant in a heavy sea where ambient and self- 
noise are high. Some characteristics not evalu- 
ated in this study are also important, such as 
the ultimte cost, size and weight, ruggedness, 
effect on the maneuverability of the boat, and 
ease of maintenance. 

5 1 COMPARISON OF MEASURED 
CHARACTERISTICS 

Beam Width and Side Lobe Reduction 

Taking up each characteristic in turn, it is 
apparent first that beam width and side lobe 
reduction are closely correlated. Beam width 
is defined as the angle intercepted by the main 
lobe at 10 db down from the maximum. Side 
lobe reduction is measured by the number of 
decibels the highest side lobe is down from the 
main lobe. Considered only for its effect on 
the listener, beam width has to do principally 
with the ease of obtaining a bearing. For a 
single target, a fairly wide beam, say ±20 de- 
grees, gives a reasonably accurate bearing in- 
dication. By sharpening the main lobe, this 
may be improved and the number of times it 
is necessary to train the beam through the tar- 


® Those interested in comparisons between various 
listening systems might also see the report on sub- 
marine and surface craft listening equipment.^ Here 
three submarine and two surface listening systems are 
given qualitative relative ratings on the basis of simul- 
taneous runs on the same targets at sea. However, 
the factors contributing to the differences found be- 
tween systems were not so completely investigated as 
in the work here described. 


get to obtain a bearing may thereby be reduced. 
However, by using a phaise-operated left and 
right indicator system with a split hydrophone, 
the accuracy for a single target becomes prac- 
tically independent of beam width. 

In the presence of an interfering source of 
sound which is not random in direction, beam 
width again becomes important. The narrower 
the main lobe, the better the ability to separate 
two targets whose intensity difference at the 
hydrophone is less than the side lobe reduction. 
When the two targets are separated by an angle 
no greater than the maximum width of the 
main lobe, resolution may always be improved 
by decreasing the width of this lobe. 

Frequency Response 

The frequency response characteristic of a 
hydrophone system is important mainly for 
the indication it gives of uniformity and phase 
balance. A so-called flat response has no sig- 
nificance, since it may be altered elsewhere in 
the circuit as desired. The existence of a reso- 
nance in the listening band may not impair the 
quality of the received signal. It does, however, 
indicate rapid changes of phase which may up- 
set the directivity patterns and the operation 
of left and right indicators, both of which de- 
pend on uniform phase relations across the 
face of the hydrophone. 

Directivity Index 

The directivity index affects the perform- 
ance of a listening system because it is a 
measure of the discrimination against ambient 
noise. Since ambient noise is always present, 
the directivity index is one of the most signifi- 
cant characteristics of a listening system. The 
effect of the directivity index of all the systems 
on ambient noise level versus frequency in a 
No. 6 sea is given in Figure 1. The results in- 
dicate an increasing improvement in the abil- 
ity to penetrate ambient noise as the frequency 




43 


44 


COMPARISON OF EXPERIMENTAL SYSTEMS 


increases. This trend is carried over to the 
high-frequency instruments and constitutes a 
point in their favor. A No. 6 sea wsls used for 


I 


50 


40 


z> 

o 

UJ 


30 

2 

0 

S 20 

1 10 


CM 0 

O 

O 

O-IO 

<n 

> 

cq-20 

o 

-30 




/ 

.HYDROPHONE WITH ZERO 
DIRECTIVITY INDEX 


9AA^^=^ 











TJH 






6-UNIT^ 

I 






6-UNIT-ELECTRICALLY STEERED 6C 

SONIC ARRAY 

7A-ELECTRICALLY STEERED - 

SUPERSONIC ARRAY 

9AA- MECHANICALLY STEERED SONIC ARRAY 

6 C -MECHANICALLY STEERED SUPERSONIC UNIT 
TTH-THROUGH-THE-HULL SYSTEM 










o 

44^ 

Z 

•54 

O 

tf) 

z 

> 

•74 Q 


.--84 


-94 


1000 


3 5 7 10,000 

FREQUENCY IN CPS 


-104 


Figure 1. Effect of directivity index on ambient 
noise of random incidence. 

this comparison because its noise level is suf- 
ficiently high to keep the output of all the sys- 
tems above their internal noise thresholds. 


to measure ambient noise with the through-the- 
hull system in less than a No. 1 sea in the band 
from 3 kc to 10 kc. The electrically steered 
sonic array is capable of measuring the noise 
of a zero sea. 

Self-Noise 

Another important characteristic of listen- 
ing systems is self-noise both at rest and under- 
way. It has been shown that hull-mounted 
sonic arrays are subject to self-noise with the 
boat at rest. In general, noises of this type 
which may be classified as hull microphonics 
have most effect on hydrophones mounted close 
to the hull and operating in the sonic frequency 
range. When the boat is under way, there are 
three primary noise sources — motion through 
the water, propeller noise, and engine vibra- 
tion. 

Relative signal-to-noise ratios have been plot- 
ted versus angle in Figure 3. At the 9-knot 
speed, the highest signal-to-noise ratio is pro- 


Internal Noise 

In order to compare the threshold curves of 
internal noise directly with ambient noise, they 
have been increased by their respective direc- 
tivity indexes as shown in Figure 2. At any 


eor 


>5 0 
o 


540 


6-UNIT -ELECTRICALLY STEERED SONIC ARRAY 
7A- ELECTRICALLY STEERED SUPERSONIC 
ARRAY 

9AA-MECHANICALLY STEERED SONIC ARRAY 
6C-MECHANICALLY STEERED SUPERSONIC 
ARRAY 


.30 


q20 


O 

O 10 
O 


m 

o. 


10 



UJ 

o 

> 

o 


1000 


3 5 7 10,000 

FREQUENCY IN CPS 


-44 

-54 

-64 

-74 

-84 


Figure 2. Internal noise in terms of equivalent 
ambient noise compared with ambient noise pro- 
duced by various sea states. 



Ui 

tn 


-1-40 

-1-30 

- 1-20 

- 1-10 

0 

-10 

-20 

-30 







1 i 1 1 

ELCOBEL — 6 KNOTS 



/ 

xIL- 

: Ln: 







^^7 

N 

. 

\ 

/ 

X 

y 1 

6C^ 

_^6-UNr 

£5 







t 

■ 



>AA 


~ 7 ^ 

ns ^ 


















6-UNIT -ELECTRICALLY STEERED SONIC ARRAY _ 
7A- ELECTRICALLY STEERED SUPERSONIC ARRAY 
9AA - MECHANICALLY STEERED SONIC ARRAY 


6C - MECHANICALLY STEERED SUPERSONIC UNIT 


•80 270“ 0* 90“ 180“ 

Figure 3. Relative signal-to-noise ratios of the 
different systems. 


frequency, the point where the system curve 
crosses a sea state curve, interpolated if neces- 
sary, defines its threshold in terms of the am- 
bient noise. For example, it would be difficult 


vided over most of the azimuth circle by the 
hull-mounted supersonic arrays. At the 6-knot 
speed, the systems show the same relative per- 
formance but with less difference among them. 









45 


COMPARISON OF SYSTEMS AT SEA 


In general, the supersonic systems are superior 
to the sonic from the standpoint of signal-to- 
noise ratio with the boat underway. 

Rear Response 

Rear response has been listed as one of the 
important attributes of a listening system. Its 
benefits, however, are limited to signals in that 
part of the azimuth circle in which there is no 
interfering noise source. As soon as the hydro- 
phone is trained toward the propellers, there 
is a rapid deterioration of signal-to-noise ratio. 


in the first part of Table 1. The estimates in 
the second part of the table are based on tests 
described in the following section. 

COMPARISON OF SYSTEMS AT SEA 

I 

The sea tests were divided into the following 
classifications. 

1. Bearing accuracy. 

2. Listening range. 

3. Effect of interference. 

4. Ease of operation. 


Table 1. Characteristics of listening systems on the Elcohel. 


System 

6-Unit 

7A 

9AA 

6C 

TTH 



A. Based on measured characteristics 


Beam width 

Fair 

Good 

Good 

Poor 

Good 

Sidp lobes 

Poor 

Fair 

Good 

Excellent 

Good 

Phase balance 

(lood 

Good 

Excellent 

Excellent 

Good 

Directivity indc.x 

(lood 

Good 

Good 

Good 

Good 

Threshold 

Excellent 

Excellent 

Pool- 

Good 

F air 

Rear loss 

Fair 

Good 

Fair 

Good* 

Fail- 

Self-noise 






Underway 

Poor 

Excellent 

Good 

Good 

Poor 

.At rest 

Fair 

Excellent 

Good 

Excellent 

Good 

Azimuth coverage 






Underway 

Fair 

Good 

Fair 

Poor 

Poor 

At rest 

Poor 

Poor 

Excellent 

Fair 

Excellent 

Resolution 

Fair 

Good 

Good 

Poor 

Good 



B. 

Based on sea tests 


Bearing accuracy 

Good t 

Goodf 

Excellent 

FairJ 

Good 

Observed range 

GoodJ 

Goodf 

Poor 

Goodf 

Excellent 

Ease of marking 

Goodf 

Excellent t 

Excellent 

Excellent t 

Good 

Ease of operation (Section 7.5) 






Underway 

Excellent 

Excellent 

Poor 

Good 

Fair 

At rest 

Excellent 

Excellent 

Fair 

Good 

Fair 


* Excellent without dome. 6-Unit electrically steered sonic array, 

t Within quadrants. 7A electrically steered supersonic array, 

t Within the azimuth coverage limits. 9AA mechanically steered sonic array. 

TTH through-the-hull system. 6C mechanically steered supersonic unit. 


The angle at which this occurs is a function of 
beam width, but for two hydrophones with the 
same beam width the signal-to-noise ratio can 
be maintained over a wider angle by using the 
boat hull as a shield against propeller noise. 

System Ratings 

On the basis of the above data and discus- 
sions, the qualitative system ratings are shown 


Most of the data were obtained with the 
Elcohel drifting and the engines turned off. 
Most of the work was done with the USNUSL 
test boat Amada as the target. 

Bearing Accuracy and Listening Range 

A summary of the bearing accuracy data for 
the five equipments is given in tabular form in 
Table 2. The bearing error is the difference 


46 


COMPARISON OF EXPERIMENTAL SYSTEMS 


between the sound operator's bearing and the 
\TSual bearing obtained with the range finder. 
A good part of the data was obtained at very 
low signal levels. The level of the target was 
purposely kept low in order to obtain data on 
maximum listening ranges along with bear- 
ing accuracy. The short ranges also reduced 
the effect of thermal conditions. 


Table 2. Bearing accuracy of Elcohel listening 
S3rstems. 


System 

Average 

bearing 

error 

(degrees) 

Standard 
deviation of 
bearing error 
(degrees) 

Number of 
observations 

TTH^ 

— 1.1 

1.76 

529 

9.\At 

-1.0 

1.45 

222 

6Ct 

+0.1 

2.17 

250 

6 Umt§ 


1.95 

353 

7AI1 


1.83 

306 

Bv quadrants 

6 Unit -1 

-1.5 

1.89 

131 

2 

+2.8 

1.80 

78 

3 

-5.3 

1.57 

41 

4 

+0.8 

2.24 

103 

7A 1 

0 

1.71 

123 

2 

-2.7 

1.90 

68 

3 

+1.0 

l.M 

47 

4 

-0.7 

2.13 

68 


• TTH throusfa-tlie-hiill system, 
t 9AA mechanically steered sonic array. 
t 6C mechanically steered snpersonic unit. 

{ 6-Unit electrically steered sonic array. 

1 7A electrically steered supersonic array. 

The phase-actuated locator [PAL] indicator 
does not remove the sources of error. Its func- 
tion is pi*imarily to assist the ear in obtaining 
a bearing and in this way to contribute to the 
overall accuracy. Some tests were made to 
evaluate this contribution. Using the electri- 
cally steered sonic array, 250 observ ations were 
made with and without the PAL. The differ- 
ence in the distributions of error is shown in 
Figure 4. This shows the increase in the per 
cent of total errors below a given value ob- 
tained by using the PAL, indicating a much 
narrower error distribution. The observed 
maximum ranges for each system are averaged 


as follows: 

Avg Max 

Sea 

System 

Range 

State 

Through-the-huU 

825 

1 

6-Unit electrically steered sonic 

700 

1 

7A Electrically steered supersonic 

800 

1 

Through-the-huU 

680 

2 

6C MechanicaUy steered supersonic 

600 

2 

9AA Mechanically steered sonic 

400 

2 


(The shortness of these ranges is of course due 
to the purposely low level of the artificial tar- 
get.) 

In another series of tests, the ship noise of 
the Amada proceeding at 5 knots was used as 



BEARING ERROR- DEGREES 

Figure 4. Improvement in bearing accuracy due 
to use of PAL indicator. 

the target for listening from the Elcohel while 
the latter was underway. A plot of the maxi- 
mum observ’ed listening range for each system 
versus the speed of the Elcohel is shown in 
Figure 5. 



Figure 5. Maximum observed range versus 
speed; signal from Amada at 5 knots; No. 1 sea. 


Effect of Interference 

The effect of target interference on bearing 
accuracy was tested with the 9AA sonic system 
and the phase-operated left and right indicator 
system under sea conditions. The auxiliary 
Decibel supplied the artificial sound source and 
a second boat, the Billie B, produced the inter- 
ference by running at a number of fixed angles 
between it and the Decibel with reference to 




COMPARISON OF SYSTEMS AT SEA 


47 


the listening boat, the Elcobel. The observers 
began to search for the target as soon as the 
interfering ship got under way. The bearings 
were recorded until the interference became 
so weak that there was no longer any doubt 
as to the location of the target ship. The two 
signals were sufficiently different in character 
so that the presence of the target could be de- 
tected soon after the interfering ship began 
its run. 



^ 1 ' ' ' ' i 



— 

/: 

1 

.1 

1 

JLj. 

1 


1 





VISUAL BEARING OF TARGET 


BEARING OF TARGET OBSERVED BY LISTENING 

VISUAL BEARING OF INTERFERENCE 


Figure 6. Resolution of target and interference 
using 9AA system. 


A plot of the bearings obtained compared to 
the true location of the target and of the inter- 
fering ship is given in Figure 6. Four runs 
are shown with the true locations of the target 
and interference as determined by visual bear- 


ings and the apparent location of the target as 
obtained with the 9AA systems. The levels of 
the interference and the target were equal at 
about 500 yards. In general, large bearing er- 
rors were obser\’ed when the level of the inter- 
ference was equal to or greater than that of 
the target. With the exception of runs 2 and 
5 the level of the interference decreased about 
4 db before the observer was sure of the target. 

Ease of Operation 

The relative ease of obtaining a bearing with 
the five different systems is determined by two 
factors. One is the physical effort required to 
train the hydrophone. The other is the type of 



NUMBER OF SWINGS TO OBTAIN A BEARING 



•racking of TRAINING SHAFT LOOSENED 

Figure 7. Time involved in obtaining a bearing. 

indicator provided to tell the obser\’er when he 
is on the target. In spite of differences be- 
tween the systems, there was no appreciable 
difference in the time (about 2 seconds) taken 
by the obser\’ers using the phase-operated bear- 
ing indicator to swing through the target. Any 
difference came in the number of times it was 
found necessary to do so, as shown in Figure 7. 



48 


COMPARISON OF EXPERIMENTAL SYSTEMS 


^ 3 CONCLUSIONS AND 

RECOMMENDATIONS 

The chief requirement for any underwater 
listening system is maximum range with suf- 
ficient bearing accuracy. All other features 
have a relative importance according to their 
contribution to this primary objective. The 
maximum observed range depends on the rela- 
tive levels of signal and noise delivered to the 
observer and on his ability to distinguish be- 
tween them. 

Only the signal level is completely outside the 
control of the system designer. The water noise 
picked up with the signal can be reduced by 
making the transducer directional. Depending 
on the frequency band, a limit for transducer 
directivity is reached either in the size of trans- 
ducer required or in the beam width, which 
becomes so narrow that a target cannot be 
found. The internal noise of the transducer 
reaches a physical limit set by thermal agita- 
tion in the resistive component of its imped- 
ance. The electric circuit has the same limi- 
tation with noise added by amplifier tubes and 
interference. Self-noise, which increases rap- 
idly as the boat gets underway, can be con- 
trolled by placing the transducer so as to shield 
it from the propellers and by shaping it or 
providing a dome so as to reduce local turbu- 
lence. Since self-noise usually originates at 
certain parts of the hull, the directivity of the 
transducer also plays a part. 

With this general background the following 
specific recommendations for listening systems 
on small patrol craft can be made on the basis 
of the experience with such systems on the 
Elcobel. 

Hydrophone 

The hydrophones should be free from uncon- 
trolled resonances in the listening band and 
should be divided into two symmetrical halves, 
each at least 3 wavelengths across at the upper 
frequency limit, for use with a phase-operated 
left and right indicator system. Sufficient ver- 
tical directivity should be included to provide 
an overall directivity index in the order of -15 
db. From the standpoint of self-noise, super- 
sonic frequencies are best, but it is recognized 


that longer ranges may be obtained at sonic 
frequencies. For the detection of submarines 
using evasive tactics, it may be of advantage 
to listen to frequencies below 1 kc, although 
the value of such detection is reduced by lack 
of directivity and high noise levels. If only 
one frequency band can be made available, the 
region from 5 to 10 kc heterodyned to a band 
below 5 kc is recommended. 

Training Mechanism 

To obtain the greatest azimuth coverage with 
the greatest accuracy for a fixed maximum 
hydrophone dimension, a mechanically steered 
transducer is recommended. If it is desired to 
use electric steering with hydrophone mounted 
in the hull, the frequency band should not ex- 
tend below 10 kc as self-noise becomes exces- 
sive. In such cases also, the location of the 
hydrophones should be carefully selected to 
minimize hull interference with the signal and 
to obtain as much shielding from the propellers 
as possible. 

Bearing Indicator 

Experience with the PAL type of bearing 
indicator leads to a definite recommendation 
that some form of phase-operated indicator be 
included in the equipment. Bearing accuracy 
is improved by this means and the number of 
times the sound operator finds it necessary to 
sweep through the target to get a bearing is 
materially reduced. With a hydrophone each 
half of which is 3 wavelengths long, targets of 
equal intensity 15 degrees or more apart may 
be separated with less than 0.3-degree error, 
using a frequency band 1 octave wide. 

Performance 

Bearing Accuracy. Tests at sea indicate 
that with a line hydrophone 3 wavelengths 
long, 65 per cent of the bearings are within 
=bl degree of the true bearing and 88 per cent 
within ±2 degrees. These are overall errors, 
including those due to the equipment, the 
observers, and the reference which was a 
visual bearing obtained by means of a range 
finder. It is felt that errors due to the equip- 


CONCLUSIONS AND RECOMMENDATIONS 


49 


merit alone were much smaller and with auto- 
matic training under the control of the phase 
indicator could be reduced to less than ±:0.5 
degree. 

Estimated Ranges. With the equipment rec- 
ommended, some estimates of maximum listen- 
ing ranges are of interest. The signal is as- 
sumed to be the sound from a submarine 
proceeding at 4 knots, periscope depth. The 
water is assumed to be free of velocity gra- 


dients, shallow (less than 100 fathoms) over a 
hard bottom. The listening boat is drifting in 
a No. 1 sea and the observer’s recognition dif- 
ferential is 6 db. Under these conditions, the 
estimated maximum listening range is 6,000 
yards for sonic frequencies and 5,000 yards for 
supersonic. For the same conditions but with 
the boat underway at 6 knots, the estimated 
ranges are reduced to 1,500 yards and 2,500 
yards respectively. 






Chapter 6 


BEARING INDICATOR SYSTEMS 


T wo CLASSES of indicators were developed in 
order to assist determining bearings with 
listening gear. The first indicates a null on 
bearing and gives deflections to right or left 
when slightly off bearing in the corresponding 
directions; the second class gives a maximum 
indication when on bearing. 

The first class includes the various types of 
phase-operated left and right indicator sys- 
tems, two of which are described below. It 
also includes the cathode-ray phase indicator. 
When used with these devices the hydrophone 
array must be divided into two electrically 
symmetrical halves having separate electric 
outputs. The relative phase of the outputs is 
then compared and the amount by which one 
leads or lags behind the other is indicated. 

Examples of the second class are the volume 
level indicator, continuous search indicator, and 
the electron ray level indicator described in this 
chapter. These devices aid the ear in determin- 
ing the bearing where the signal is loudest. 

Two meter-actuating circuits exemplifying 
the characteristics of phase operated left and 
right indicator systems are discussed : the 
phase-actuated locator [PAL] and the vector 
bearing indicator [VBI].^ One other system of 
this kind was tested on the Elcobel and is 
described as the cathode-ray phase indicator. 
However, this latter system was found to be 
inherently more expensive and more difficult to 
maintain. Further, the indication presented on 
the face of the cathode ray tube was too faith- 
ful a copy of the unwanted noise peaks which 
confused the left and right indication. It was 
therefore discarded in favor of the meter cir- 
cuits of the type discussed here, which have a 
considerable advantage in simplicity and cost. 

The PAL and VBI circuits represent two 
versions of the fundamental principle of con- 
verting phase differences into level differences. 


Their operation depends upon the properties of 
the hydrophone used. Some hydrophone direc- 
tivity equations will be developed before taking 
up the indicator systems. 

HYDROPHONE RESPONSE 
FORMULAS 
General Equation 

It is assumed throughout the discussion that 
the hydrophone has two identical halves, both 
of which are controlled as to resonances within 
the passed band and each of which has similar 
directive properties. They are also assumed to 
be mounted in the same plane, symmetrically 
with respect to the center line of the combina- 
tion. The most general case is one in which the 
sensitivity varies along the face of each hydro- 
phone as a function of x, the distance from the 
center line along the horizontal axis. The out- 
put of either hydrophone at a given frequency 
for any angle of incidence ^ of a sound wave of 
unit amplitude is given by 

^ ±Z2 

E= /(x)e’'“‘ + “’dx, (1) 

J ±X1 


(NORMAL TO HYDROPHONE) 



Figure 1. Diagram of wave front passing hydro- 
phone. 


® The VBI circuit described here is essentially iden- 
tical with the right-left indicator [RLI] circuit em- 
ployed by the New London Laboratory and described 

in Chapter 10. 

* 


where f {x) is the law of variation of sensitivity 
and a is the phase delay of the wave, along the 
X axis. Referring to Figure 1 which shows the 



50 


NULL INDICATORS 


51 


hydrophone with the normal Nh to its face and 
the wave front with its normal the distance 
the wave has traveled past -f a: at the time it is 
passing through zero is 

X sin e, 

and the time required to do so was 

X sin d 
c 

where c is the velocity of sound in the medium. 
The phase angle may therefore be written 

a)X sin d 27 rx . ^ 

cx = = — — sin d, 

c X 

where A is the wavelength. Dividing by the 
integral of the sensitivity function will refer 
the output at any angle 0 to the output at 0=0, 
thus 


f(x)e^^^ + X dx 



Special Cases 

A special case is shown by Figure 2. Two 
hydrophones of length L are used, each of 
which has a symmetrical taper about its center. 



Figure 2. Pair of symmetrically tapered hydro- 
phones. 


The output of either one of the pair can be 
reduced to the product of its directivity func- 
tion F(L/x,0) about its own center and the 


phase delay of its center with respect to the 
midpoint of the pair, thus 

E = Fe (3) 

This can be done because the acoustic center of 
a symmetrically tapered line is always coin- 
cident with its physical center. 

An even simpler case is when the taper is 
zero and each hydrophone is a uniform line of 
length I with thfe two joined together so that 
their respective centers are at ±1/2. The di- 
rectivity function F for each half of such a 
hydrophone is deducible from formulas given 
in standard texts. 



The response then takes the simple form 



62 null indicators 

PAL Indication 

The phase-operated left and right indicator 
system used with the electrically steered sonic 
and supersonic arrays and with the mechani- 
cally steered 9AA and 6C hydrophones described 
in Chapter 2 is a development of the PAL sys- 
tem for harbor protection. A block schematic 
showing the essential elements of the circuit is 
given in Figure 3. The output of each hydro- 
phone is fed through two independent ampli- 
fiers which, in practice, have automatic volume 
control [A VC] to maintain constant levels at 
their outputs. Identical filters are used to limit 
the frequency band, and phase shifting net- 
works produce a 90-degree shift in one channel 
relative to the other before the sum and differ- 
ence are obtained in the mixing circuit. The 
sum and difference are rectified and the d-c 
output of each detector is fed in opposition into 
a center zero meter. The effects of A VC will be 
omitted temporarily in the following discussion. 




52 


BEARING INDICATOR SYSTEMS 


Symmetrically Tapered Hydrophones. Exam- 
ining each channel separately and including 
the 90-degree phase shift between channels, we 
get from (3) for two symmetrically tapered 
directional lines, 

Loft channel Ej =Fe^^ 

Right channel Ej^ = Fe^^ ^ . (5) 


It is now assumed that the detectors are of 
the square law type. While this is not always 
the case in practice, the results agree closely 
with more elaborate computations on linear 
detectors and with experiments using a band 
of thermal noise. The rectification process, 
therefore, squares the real part of the sum and 
difference expressions above, and the meter 



SUM 

DETECTOR 



DETECTOR 


DIRECTION OF 
SIGNAL 


NORMAL TO 
CENTER LINE 


INPUT 



90* PHASE 
SHIFT 



AFTER 

MIXING 



AFTER 

RECTIFICATION 


/■(A + B)-/(A-B) = 0 


METER 

INDICATION 


(NO DEFLECTION) 


FROM LEFT 





(DEFLECTS LEFT) 


FROM RIGHT 





/(A + B)-/(A-B) = + (DEFLECTS RIGHT) 


Figure 3. Block diagram of PAL circuit. 


The sum of these two channels is now ob- 
tained in the mixer circuit. 


S 


sin 6) i( sin 8 + -| ) 

g X e ^ ^ 


= F j^(cos a — sin a) + j{cos a — sin «) J . 
Similarly, the difference 

= F |^(cos a + sin a) — j{cos a + sin a) J 


D 
where 


( 6 ) 


(7) 


tt/ sin d 


response is the result of opposing the d-c por- 
tions of the detector outputs, i.e. : 


(Re >S)2 = F2 |^(cos co/ — sin cof)(cos a — sin a) , 

of which the d-c portion is : 

(Re >S)dc = F^ (cos a — sin a) 2, 
and similarly for the difference. 


(8) 

( 9 ) 


(Re D)dc = E2 (cos a + sin a)^. 

Hence the meter current is : 

I = (Re ^S)dc - (Re D)L = - 2F^ sin 2a. (10) 


NULL INDICATORS 


53 


Unifoi'm Line Hydrophones. For two con- 
tiguous uniform lines, equation (10) becomes 


/ =- 2 


sin sind) 
A 

tI . 

— sin 0 
A 


2Trl 

sin ( sin 6) . ( 11 ) 

X 


The results of the computation for the uniform 
lines are given in Figure 4, curve A. The 
ordinate is the current through the meter and 
the abscissa is given in terms of a = (A sin 6) /A, 
which is measured in radians so that the curve 



a 

X 


Figure 4. PAL response with directional hydro- 
phones (single-frequency signal). 


is applicable to lines of any length I for any 
angle of incidence 6. It should be stated that the 
response follows a similar but inverted curve 
below the axis on the left side of the null point. 

Effect of a Band of Frequencies. The discus- 
sion so far has been concerned with a signal 
consisting of a single frequency. The sound 
from ship propellers consists of a band of fre- 
quencies. Assume for the moment that this may 
be considered a band of noise whose single-fre- 
quency components all have the same intensity 
at the hydrophone. The meter current is given 
by an integral of the form 



where the factor preceding the integral serves 
to normalize it. For two uniform lines, after 
substituting from equation (11), this becomes 




Ci(2e,) - Ci(20,) - Ci(4fli) 
-f Ci(4e.) 4- 


+ 


' 20, 

sin (4^2) sin {ASy 

4$, 


46 . J ’ 


(13)" 


where 



6 


and 62 = 


CO 2/ 


sin 6, 


and c = velocity of sound in the medium. 



0 12 3 4 5 

d IN RADIANS 

ttI sin e 

OC = 

X 

Figure 5. PAL response with uniform line hydro- 
phones (wide-hand signal). 


The results obtained by keeping the upper 
frequency fixed while the lower frequency of 
the band is varied are shown in Figure 5. The 
curves represent bandwidths of approximately 


^ Ci(a) is the “cosine integral,” Cifa) 



whose values are listed in the standard tables of functions. 


CQJXFJDENTjA L 


54 


BEARING INDICATOR SYSTEMS 


1 / 2 , 1, and 2 octaves. The abscissa is again given 
in radians and applies to the upper frequency 
limit. As the band is widened by reducing the 
lower frequency, the discrimination becomes 
poorer at most values of the abscissa because of 
the decreased directionality of the hydrophones 
at the lower frequencies. 

When the hydrophone is trained on the 
target the meter of the indicator reads zero. 
The meter may be poled, either to show on 
which side of the hydrophone axis the target 
is located (left and right indicator) or on 
which side of the target the hydrophone is 
trained. As the hydrophone is trained past the 
target, the meter may swing through zero a 
number of times. For instance, for curve A in 
Figure 5 there is a crossing at a = 1.9 radians. 
This, however, is in the opposite direction to 
the main crossing at zero and is also very un- 
symmetrical as regards the right and left ex- 
cursion of the needle. It can easily be differen- 
tiated, therefore, from the true zero. The next 
crossing is at a = 3.7 radians and is in the same 
direction as the main crossing, but excursions 
on either side of the zero point are so small 
that there would be no difficulty in identifying 
it as a false zero. This becomes increasingly 
true of crossings at higher angles. 

Effect of AVC. It now becomes necessary to 
consider the effect of AVC on these patterns. 
There are two reasons for its use. It removes 
all effects of level changes except those which 
are proportional to the phase difference in the 
two channels. The meter deflection then be- 
comes a measure of the number of degrees off 
bearing. Secondly, it removes the necessity for 
manual volume adjustment in order to keep the 
meter deflection within the proper range, not 
only for the observer’s convenience in reading 
it but to insure that the detector is being loaded 
to best advantage. The disadvantage of AVC 
is that it removes the level difference between 
lobes in the pattern of the hydrophones. The 
hydrophones, therefore, become essentially non- 
directive. 

Effect of an Interfering Signal. The effect on 
the meter indication of bringing another sound 
source into the field can be evaluated by the 
principle of superposition, that is, by obtaining 
the pattern for each source separately and 
adding. A number of patterns, computed for 


the 9AA hydrophones at different relative 
levels of target and interference are given in 
Figure 6 for an angle of 15 degrees between the 
target and the interference. AVC does not 
enter this calculation because the two signals 
go through the amplifiers at the same time and 
the gain is the same for both. When the two 



-20 -10 0 10 20 30 40 50 

e IN DEGREES 

BOTH TARGET AND INTERFERENCE EMITTING A BAND 
OF NOISE FLAT FROM 2 -IOKC 

Figure 6. Effect of interference in field of target 

sound. 

signals are of equal intensity, they tend to pull 
their respective zero crossings toward each 
other by an equal amount which constitutes the 
error in their respective bearings. When they 
are 2 db or more apart, the stronger of the two 
takes over and no bearing can be obtained on 
the weaker. 

Effect of Ambient Noise. The ability of bear- 
ing indicators to operate below ambient noise 
would appear to depend upon the character of 
the noise. If the distribution is random in 
angle and the time variation, averaged over a 
suitable interval, is small, a positive indication 
is obtained even though the average signal level 
is below the noise. In such cases the noise level 
controls the amplifier gain through the AVC 
which reduces the sensitivity for the signal. 
Removing the AVC would not improve matters 
because the noise would then overload the 
detectors, whose balance cannot be maintained 
over a very wide range of levels. 

Since the meter attempts to follow isolated 
noise peaks which may be directional, the ob- 


C< 


L 


NULL INDICATORS 


55 


served decrease in the standard deviation of 
ambient noise with increased depth should 
work to the advantage of this type of indicator 
on a submarine. Although there have been in- 
stances where a bearing could be obtained with 
the meter beyond the range at which listening 
alone was effective, there is no positive evidence 
that its recognition differential is superior to 
that of the ear. 

Effect of Inaccuracy in the Phase-Shift Net- 
work. Let us consider the behavior of the cir- 
cuit over small angles in the vicinity of the null 
point so that the directivity function F may be 
omitted. A single frequency is also assumed. 

The net gain from hydrophone to mixer cir- 
cuit is* designated by A for the left-hand chan- 
nel and B for the right. The relative phase 
shift between the channels is <^. The inputs to 
the mixer circuits are then : 

Left channel El = 

Right channel Er = (14) 

The output of the sum detector is proportional 
to the real part of the sum squared ; expanding 
and using only the d-c terms, we get 

B^ 

(Re <S)dc ~ ~2 '^~2 

+ AB (cos 2a cos 0 — sin 2a sin </>). (15) 

Similarly, the output of the difference detectors 
is given by 

, A2 ^2 

(Re D)do ~ 2 2 

— AB (cos 2a cos </> — sin 2a sin </>). (16) 

Since these two currents are passed through 
the meter in opposition, the net meter current 
is their difference, which is : 

I = 2AB (cos 2a cos </> — sin 2a sin 0). (17) 

Since A and B are finite, the current becomes 
zero when 

cot (f) = tan 2a. (18) 

For this to occur when a = 0, must equal 90 
degrees. This is the reason for the fixed 90- 
degree phase shift. 

Any departure from 90 degrees causes the 
null to shift to a new value of a, say a', and a 
faulty bearing is obtained. Taking the differen- 
tial of (18) : 

— csc2 = sec2 (2a')d(2a'), (19) 


where a is the angle at which 7 = 0. When <f> is 
close to 90 degrees, csc^ <f> approaches unity. 
When a is close to 0 degrees sec^ a approaches 
unity, hence for small departures from the 
desired values the variation of a' with <f> is : 


d {2a) = d<f) = — cos e', de', (20) 

1 

where 6' is the bearing angle at which 7 = 0. 

Since a and therefore 6' were assumed to be 
small. 


and 



dd' X 
d(f) * 


(21) 


The longer the line in wavelengths, the 
smaller the bearing error for a given deviation 
of the fixed phase delay from 90 degrees. 

It is apparent from equation (17) that gain 
variations, including the response of the hydro- 
phones, merely change the magnitude of the 
meter deflections but do not affect the bearing 
of the null indication. If the product of the 
gains, AB, can be maintained constant, there is 
no change at all in the amount of meter de- 
flection. For this purpose A VC is used, with 
the result that the meter deflection varies only 
with the number of degrees off bearing. 

Inasmuch as the contributions from various 
frequency components to the total direct cur- 
rent is proportional to the square of the volt- 
ages, the effective phase shift error of the cir- 
cuit over a band of frequencies can be found 
from 



e2(co) • A<f){cS) • do3 


e2(co) • dco 


( 22 ) 


which should cover a frequency range some- 
what above and below the pass bands of the 
Alters. 


VBI Indication 

There are a number of different circuit ar- 
rangements which can be used to produce a d-c 
indication of bearing depending on the phase 
relations of the output of the two hydrophones. 


56 


BEARING INDICATOR SYSTEMS 


The VBI circuit is shown in Figure 7. The 
first operation is mixing to obtain the sum and 
difference. Then the 90-degree phase shift is 
introduced, after which the two channels are 
combined in the detector which is essentially 
another mixer plus a rectifier. Those character- 
istics of VBI which depend on the directivity 
of the hydrophones do not differ appreciably 


phase shift (j> is introduced into the difference 
channel thus 

S = C{Ae-’- + 

D = D{Ae-i'‘ - ^ ’ 

The real parts of these sum and difference 
voltages are combined in a multiplying detector 
and the d-c output is passed through a center 



DIRECTION OF 
SIGNAL input 



AFTER 

MIXING 



90* PHASE 
SHIFT 



AFTER 

RECTIFICATION 


/(C+D)-/ (C-D) = 0 


METER 

INDICATION 


(NO DEFLECTION) 


FROM LEFT 



FROM RIGHT 






/(C + D)-/(C-D) = + 


/(C+D)-/(C-D)=- 


( DEFLECTS LEFT) 


(DEFLECTS RIGHT) 


Figure 7. Block diagram of VBI circuit. 


from those of PAL which have already been 
discussed. A principal advantage of VBI is a 
decreased sensitivity to inaccuracy in the phase 
shifts within the circuits. 

Effect of Inaccuracy in the Phase-Shift Net- 
work. For the VBI circuit, the voltage outputs 
of the two hydrophones are again written in 
complex form: 

Left hydrophone El = Ae 
Right hydrophone Er = 

The voltages are then fed into a mixer circuit 
which forms the sum and difference ; these pass 
through amplifiers with gains C and D, and a 


zero meter. The multiplying detector is really 
another mixer with square law detectors whose 
outputs are in opposition. It gets its name from 
the fact that 


Re (S 4- Dr - Re {S - Z))2 = 4 Re (S) Re (D) 
= CD [{A Br sin a cos a cos oit sin (cof -f 0) 
-h(A — R)2 sin a cos a sin cof cos 4- <f>) 
4-(A2 — B^) cos2 a cos cot cos (cot 4- 0) 

4- (A 2 — R 2 ) a cos cot sin (cot 4- 0)]. 


After expanding and dropping the a-c terms, 
the expression for the meter current is 


I = CD AB sin 2a sin 0 4- 


A2 - 52 


cos 




(25) 


2 



NULL INDICATORS 


57 


This discloses the following characteristics of 
the VBI circuit: 

1. The bearing at which a null is obtained 
is not affected by level differences in the two 
channels ; only the magnitude of the deflection 
on each side of the null is changed. This is the 
same as for the PAL circuit. 

2. When the two hydrophones feed equal volt- 
ages to the mixer circuit (A = B ) , changes in <j> 
do not affect the bearing at which a null is ob- 
tained. This differs from the PAL circuit 
[equation (18)]. 


Near 4 > = 90^, esc- </> approaches unity. Near 
a' = 0°, cos^ 2a' approaches unity. Hence for 
small departures from these values : 

42 _ D2 1 / 

— 2 XJf~ ^ d{2a') = — cos (d')d{e') . 

Since $ is also small whqn a' is near zero. 

d<j> ^TTl \ 2AB ) 

The term in parenthesis represents a factor 
which, for small differences in hydrophone re- 



Figure 8. Block diagram of cathode-ray phase indicator. 


3. When A differs from B and (f> differs from 
90 degrees the null is shifted by an amount 
which can be determined by equating the cur- 
rent to zero, giving : 


sponse, makes the VBI circuit less sensitive 
than the PAL circuit to phase-shift errors in 
the amplifiers, networks, and filters of the sum 
and difference channels. 


A 2 TJ2 

sin 2a’ = - cot <t, • (26) 

Differentiating this, leaving A and B con- 
stant. 


cos^2ad(2a') = csc 2 


A2 - B2 


d(f) . 


(27) 


Cathode-Ray Phase Indicator 

An oscilloscopic method of bearing determi- 
nation, which has been used for other applica- 
tions, was also tried with the steerable arrays. 
The pattern obtained on the oscilloscope con- 




2AB 



58 


BEARING INDICATOR SYSTEMS 


sists of a horizontal line which has two movable 
vertical lines extending from it, one below and 
the other above. When on the correct bearing, 
the two vertical lines form a single line in the 
middle of the pattern. When off bearing these 
lines move apart in a direction which depends 


factory performance obtained with the PAL. 
Figure 8 illustrates the principle of the cath- 
ode-ray phase indicator. The outputs of the 
two halves of the hydrophone array are first 
passed through band-pass filters and amplifiers 
having A VC. The amplifiers feed into modula- 


0.1 MF 0.1 MEG 



upon the direction of the error. Satisfactory 
performance was obtained with single-fre- 
quency inputs but for a band of noise several 
kilocycles in width the pattern moved rapidly 
in an irregular fashion so that it was difficult 
follow. At this point further work on the 
equipment was abandoned in view of the satis- 


tors which are supplied from two oscillators 
whose frequencies differ by 1 kc. The modulator 
outputs then pass through an 8- to 12-kc filter 
on the lower side and a 9- to 13-kc filter on the 
upper channel which isolate the lower side- 
bands from the other modulation products. 
After this they are fed into a third modulator 


MAXIMUM INDICATORS 


59 


which yields a difference frequency of 1 kc. The 
oscillators’ outputs are also passed into a fourth 
modulator which yields another difference fre- 
quency of 1 kc. However, these two 1-kc out- 
puts differ in phase by the amount existing at 
the input terminals. The output of modulator 4 



Figure 10. Arrangement for indicating bearing 
while the hydrophone rotates. 


is applied to the horizontal plates of an oscillo- 
scope, thus providing a sinusoidal sweep. 

The output of modulator 3 could also be ap- 
plied to the vertical plates directly and thus 
produce the well-known Lissajous figures which 
would, vary from a straight line having an 
angle of 45 degrees for zero phase shift to a 


circle for a 90-degree phase shift. However, a 
better indication is obtained in the present 
device by passing the 1-kc difference frequency 
obtained from modulator 3 through a limiter, 
which supplements the A VC, and then through 
a power amplifier to a Peterson coil having an 
easily saturated core. At the beginning of each 
half-cycle this coil is not saturated and a volt- 
age is impressed across the vertical plates of 
the oscillator for a short time. This occurs at 
the moment that the horizontal plate is in the 
middle of the line when there is no phase shift, 
but at an earlier or later time if the phase is 
leading or lagging. After this brief interval in 
which the coil is not saturated, the increasing 
current causes the inductance to drop very 
close to zero, so that no further voltage appears 
across the oscilloscope plates until the current 
again nears zero at the end of the cycle. 

MAXIMUM INDICATORS 
Electron Ray Level Indicator 

This device is a maximum indicator designed 
to replace the volume level indicator with a 
magic-eye cathode-ray tube. It has the ad- 
vantage of wide ranges in adjustment for the 
rates of closing and opening the eye. The cir- 
cuit arrangement shown in Figure 9 consists 
of a two-stage feedback amplifier followed by a 
full wave indicator which supplies direct cur- 
rent to the grid of the magic-eye tube. The in- 
put of the amplifier is of high impedance so 
that it can be bridged across a 600-ohm circuit. 
Between the amplifier output and the full wave 
indicator, there are transformers to permit the 
insertion of 600-ohm filters. The overall re- 
sponse is fiat within ±2 db from 100 c to 20 kc. 
A full closure of the eye is obtained on 40 db 
below 1 volt. The time constant provided is ap- 
proximately 1/5 second. 

Continuous Search Indicator 

Continuous rotation of the 6C hydrophone on 
the Elcobel was made possible by means of slip 
rings attached to the upper end of the shaft. 

As shown in Figure 10, an arrangement is 
provided for indicating on a persistent scre^ 
oscilloscope the bearing of a sound source while 


60 


BEARING INDICATOR SYSTEMS 


the hydrophone rotates. The output of the two 
halves of the hydrophone are connected in 
parallel, amplified and heterodyned down to the 
audio range, and then fed to the Z amplifier of 
this oscilloscope which controls the intensity of 
the spot. The position of the spot is determined 
by a special potentiometer. The brushes are 
attached to arms at right angles to each other 
and connected to the deflecting plates of the 
oscilloscope through two slip rings. 

As the hydrophone is rotated by a belt drive 
from a motor and gear reduction box, the volt- 
age applied to the deflection plates varies si- 
nusoidally, the horizontal plates being 90 
degrees out of phase with the vertical plates so 


that the spot is moved in a circular path. Since 
the intensity of the spot is controlled by the 
hydrophone output, a bright section of the cir- 
cumference can be seen on the screen when the 
hydrophone passes through the bearing of a 
sound source. Random noise appears as scat- 
tered dots. Although the locations of sound 
sources are shown on the oscilloscope screen, 
the traces are rather broad, since they follow 
the beam width of the transducer and isolated 
noise peaks contribute meaningless bright 
spots. It was found that some means were 
needed for sharpening the indication as well 
as a method for discriminating against random 
noise peaks. 



Chapter 7 


SURFACE CRAFT LISTENING EQUIPMENT— 
JP SYSTEMS 


INTRODUCTION 

T he small craft engaged in coastal patrol 
work consisted mostly of converted private 
yachts and fishing vessels about 50 feet to 100 
feet in length. Because of their small size and 
inadequate armament, it was the primary func- 
tion of these vessels to report the location of 
enemy submarines rather than to attack them. 
Echo ranging was not feasible from ships of 
this size, particularly with the type of equip- 
ment in existence at that time, nor was suffi- 
cient gear available to equip the small craft for 
this function. Sonic listening appeared to be 
practicable for patrol craft search of sub- 
merged submarines and submarines at night. 

It was believed that to be useful in detecting 
submarines from small patrol craft such sonic 
equipment should have a listening range of 
several thousand yards under good conditions 
and should be capable of indicating the target 
bearing to within a few degrees. The mechani- 
cal part of the gear should be easily adjustable 
to accommodate installation on widely varying 
types and sizes of vessels, and its operation 
should not contribute appreciably to the back- 
ground noise level. 

On this basis, two types of directive sonic 
detectors were developed, an overside equip- 
ment and, later, a through-the-hull equipment. 
To simplify production and to avoid the neces- 
sity for drydocking the vessels during installa- 
tion, an overside type of gear was chosen for 
primary development in spite of the mechanical 
and hydrodynamical advantages of a through- 
the-hull design. 

Design Principles 

The requirements demanded a hydrophone 
having sufficient efficiency to insure an adequate 
signal to resistance-noise ratio in the sonic 


region® and sufficient size to provide reasonably 
sharp directivity in the upper sonic frequen- 
cies. To avoid ambiguity between reciprocal 
bearings, the hydrophone selected had a front- 
to-back discrimination of at least 10 db over a 
broad frequency range. 

The amplifier was designed to have a uniform 
frequency response from about 0.1 kc to 10 kc. 
Several supplementary high-pass filters were 
provided to permit progressive exclusion of the 
lower frequencies, when desired, in order to 
discriminate against certain types of back- 
ground noise and to take advantage of greater 
hydrophone directivity at the higher frequen- 
cies. To supplement loudspeaker and head- 
phones, an indicator such as a magic-eye tube 
was found desirable in aiding determination of 
bearings. 

The mounting and training gear were de- 
signed to locate the hydrophone an appreciable 
distance below the keel to avoid acoustic shield- 
ing by the hull and to provide for easy and ac- 
curately controlled rotation of the hydrophone 
from a training wheel which was located in the 
deckhouse. 


Uses of Experimental Gear 

Fifty preproduction units of the overside 
gear were installed for tests and training pur- 
poses. A total of 1,500 units was produced, but 
they were not placed in service because of the 
removal of enemy submarine activity from 
coastal waters. However, the design principles 
and experience gained were utilized in the 
development of topside directive sonic listening 
gear used on U. S. submarines and discussed in 
Chapter 10 of this volume. 


a For a 1 cycle wide band, minimum measurable 
pressure not greater than -34 db vs 1 dyne per sq 
cm at 0.1 kc, -54 db at 1 kc, and -74 db at 10 kc. 




61 


62 


SURFACE CRAFT LISTENING EQUIPMENT JP SYSTEMS 



Figure 1. Hydrophone and baffle for JP overside 
equipment. 


JP Overside Equipment 

The JP overside equipment is designed to he 
used on small patrol craft to pick up the under- 
ivater sounds of submarines and indicate their 
direction from the ship. The equipment consists 
of a directional toroidal magnetostriction hydro- 
phone, a sonic amplifier with battery power sup- 
ply, and a training mechanism. The hydrophone 
is mounted on a shaft extending into the water 
over the side of the vessel. Because of the 
method of suspension, the system can be used 
to advantage only in relatively calm seas. The 
listening vessel must be lying to with all ma- 
chinery secured. The hydrophone response rises 
with frequency at the rate of about 3 db per 
octave from a value of —115 db vs 1 volt per 
dyne per square cm at 1,000 c. The amplifier 
has a flat frequency characteristic in the range 
0.2 kc to 10 kc and is equipped with a series of 
four high-pass filters cutting off at 500 c, 1,500 
c, 8,000 c, and 5,500 c. This equipment was de- 
veloped by CUDWR-NLL. 

7 2 PRELIMINARY WORK 

A bidirectional toroidal magnetostriction 
hydrophone^’ developed earlier was selected as 

^ Magnetostriction hydrophone developments are dis- 
cussed in Division 6, Volume 13. 


the type of available unit most readily adapta- 
ble to the requirements of the directional sonic 
listening equipment. Various types of acoustic 
baffles, including foam rubber, waterproofed 
cellular fiberboards, a heavy slab of litharge- 
impregnated rubber, and an iron ring with 
cork-rubber backing were tried as means of 
making this hydrophone unidirectional to per- 
mit unique bearing determinations. Of these, 
the last two met both the structural and 
acoustic requirements, and the iron ring with 
cork-rubber backing was selected because less 
rubber was required. 

An ordinary rubber hose-type coupling, tried 
as a means of providing the necessary flexibility 
to allow the overside hydrophone support to 
hang vertically in the water, was rejected when 
tests showed it to be inadequate during heavy 
rolling and pitching. A gimbal type of joint 
proved superior to the hose but still permitted 
the hydrophone shaft to depart considerably 
from the vertical in the presence of strong 
winds and tides. As no simple means of elimi- 
nating this deflciency in the overside gear was 
apparent, the gimbal type of coupling was 
adopted because of lack of further development 
time. 


7 3 FINAL OVERSIDE EQUIPMENT^^ 

Hydrophone and Baffle 

The overside listening equipment hydro- 
phone is a magnetostriction unit using 2-inch 
outside diameter nickel tubing curved to the 
shape of a toroid 24 inches in diameter. It is 
normally bidirectional, with major response 
lobes along the axis of the toroid, and has a 
sensitivity which increases with frequency at 
the rate of about 3 db per octave from a value 
of -115 db vs 1 volt per dyne per sq cm at 
1,000 c. The acoustic baffle consists of a i4-inch 
thick flat iron ring 3 inches wide backed with a 
V 2 -inch layer of cork-impregnated rubber. This 
provides a front-to-back discrimination of 10 
db to 15 db at frequencies of 1 kc and higher. 
The baffle is mounted on the training shaft and 
the hydrophone is secured to it by means of 
U-shaped molded rubber clamps which aid in 
isolating the hydrophone from mechanical 


O 




FINAL OVERSTRIDE EQUIPMENT 


vibration of the shaft. When not in use, the 
shaft is swung alongside and the whole assem- 
bly pulled inboard so that the hydrophone may 
be stowed on deck. 

Hydrophone Shaft and Training 
Mechanism 

The hydrophone shaft, of 2-inch standard 
pipe, contains concentric rubber-in-shear shock 


Figure 2. Overside equipment gimbal-type yoke. 

mounts at top and bottom to minimize trans- 
mission of vibration incident to use of the 
training mechanism. The top of the shaft is 
secured to a cast bronze gimbal-type yoke (Fig- 
ure 2) which allows the shaft freedom to swing 
in two directions. The gimbal is connected to 
the inboard mechanism (Figure 3), which in- 
cludes a training wheel and azimuth indicator, 
by means of a second length of 2-inch standard 
pipe. Through this pipe are run Mo-inch stain- 
less-steel stranded cables to rotate the hydro- 
phone shaft in synchronism with the motion of 
the training wheel. 

Amplifier and Power Supply 

The amplifier is of the impedance- and re- 
sistance-coupled type employing six tubes. It 
has a voltage gain of approximately 115 db and 


a uniform frequency response over the range 
0.2 to 10 kc. The circuit includes four high-pass 
filters cutting off at 500, 1,500, 3,000, and 5,500 
c immediately selectable by means of a rotary 
switch. High-quality headphones are used for 
listening and an electron-ray indicator tube 
(magic-eye), associated with the 5,500-c filter 
position is provided for determining bearings 
more accurately than by listening alone. 

The power supply for the amplifier is inde- 
pendent of the ship’s supply. It consists of a 
6-volt storage battery for the filaments plus 
four heavy-duty 45-volt B batteries. A 480-/xf 
bank of condensers, charged by the 180-volt 


Figure 3. Overside equipment inboard assembly. 

plate supply and discharged through the coil 
of the hydrophone, furnishes a peak current of 
approximately 13 amperes for remagnetizing 
the nickel tube. 



64 


SURFACE CRAFT LISTENING EQUIPMENT JP SYSTEMS 


Performance 

In general, the overside equipment performs 
satisfactorily only in relatively calm seas and 
with moderate winds. Under good conditions, 
ranges of 3,000 yards or more and bearing ac- 
curacies of ±21/2 degrees can be obtained. In 
rough water or strong winds, however, per- 
formance of the overside equipment is seri- 
ously impaired, owing primarily to failure of 
the hydrophone shaft to remain vertical in the 
water and to the added water noise caused by 
the violent motions of the hydrophone. 



Figure 4. JP through-the-hull equipment. 

JP Through-the-Hull Equipment 

The JP through-the-hull equipment, developed 
by CUDWR-NLL, is used by small patrol craft 
to pick up submarine sounds and indicate their 
relative direction. The equipment consists of a 
3-foot toroidally wound magnetostriction line 
hydrophone ivith baffle, sonic listening amplifier, 
battery power supply, and training mechanism. 
The hydrophone is mounted on a shaft which 
extends through the hull and can be raised for 
stowing or lotvered for listening. The system can 
be used while the vessel is under loay at 3 or U 
knots under sail, with all machinery secured. The 
hydrophone with its baffle is directional and its 
response rises with frequency at the rate of about 
6 db per octave from — 110 db vs 1 volt per dyne 
per square cm at 1,000 c. Amplifier and power 
supply are those used ivith the JP overside 
equipment. 


After completion of the overside equipment, 
development was directed toward design of a 
through-the-hull type of training mechanism 
to overcome the mechanical shortcomings of the 
overside gear and provide for continuous under- 
way listening at slow speeds. Because a sea 
chest to house the hydrophone when the vessel 
was moving at high speed could not be accom- 
modated by most of the small patrol craft, it 
was necessary to provide a unit having less 
drag than the toroidal hydrophone used with 
the overside gear. For this purpose a 3-foot 
long, straight magnetostriction hydrophone of 
wooden core construction® was selected. 
Equipped with a streamline air column baffle, 
this unit provides minimum drag together with 
good directivity and front-to-back discrimina- 
tion. 

Calculation showed that, at 8 knots, a 31 / 2 - 
inch diameter shaft is required to withstand 
the drag of the straight hydrophone and baffle 
mounted 5 feet below the hull. Later tests indi- 
cated, however, that the hydrophone could be 
mounted as close as 30 inches to 36 inches 
below the hull without impairing performance. 


7 ^ FINAL THROUGH-THE-HULL 
EQUIPMENT 

The through-the-hull equipment^ utilizes the 
same power supply developed for use with the 
overside gear. The hydrophone and the me- 
chanical arrangements differ, and the amplifier 
is modified to operate either on batteries or on 
the 110-volt d-c ship supply. 

Hydrophone and Baffle 

The hydrophone and baffle assembly used 
with the final model of the through-the-hull 
equipment is shown in Figure 5. The hydro- 
phone, a 3-foot long, plastic-covered toroidally 
wound magnetostriction uniU has less sus- 
ceptibility to magnetic interference from gen- 
erators, vibrators, dynamotors, etc. than the 
conventionally wound straight magnetostric- 
tion unit. Its sensitivity increases with fre- 

Magnetostriction hydrophone developments are dis- 
cussed in Division 6, Volume 13. 


FINAL THROUGH-THE-HULL EQUIPMENT 


65 



Figure 5. Through-the-hull hydrophone and baffle 
assembly. 



Figure 6. Through-the-hull hoist-train assembly. 


quency at the rate of about 6 db per octave* 
from a value of — 110 db vs 1 volt per dyne per 
sq cm at 1,000 c. The lowest measurable pres- 
sures with this unit for a 1-cycle band at 0.1 kc, 

1 kc, and 10 kc are respectively —62 db, —74 
db, and —75 db vs 1 dyne per sq cm. The baffle 
consists of a streamline, free-flooding, hollow 
bronze casting covered on the backside with 
a non-intercommunicating cellular rubber 
blanket. This construction was found to give 
better front-to-back discrimination than the 
non-free-flooding air column baffle used for- 
merly. 


6, which illustrates the method of installation. 
The support tube, or well, consists of 4-inch 
standard iron pipe and the training shaft is of 
3-inch seamless-steel tubing plated with suc- 
cessive layers of copper, nickel, and chromium. 
The shaft is secured at the top to the training 



Figure 7. Through-the-hull inboard assembly; 
shaft in raised position. 


Hoisting-and-Training Mechanism 

handwheel and bearing-dial assembly which 
The through-the-hull mechanical arrange- rotates in a ball-type bearing. At the bottom 
ment is shown in the sectional drawing, Figure the shaft is supported laterally by a graphited 

r 





66 


SURFACE CRAFT LISTENING EQUIPMENT JP SYSTEMS 


asbestos sleeve bearing which requires no ma- 
chining of the smoothly plated shaft. 

The hoisting mechanism for raising and 
lowering of the hydrophone consists of a rack 
gear milled into the training shaft, meshing 
with a pinion gear which is turned by a de- 
mountable hand crank. A safety lock prevents 
raising of the shaft unless the hydrophone is 
oriented fore and aft with respect to the ship. 

The bearing dial moves with the training 
shaft so that relative bearings are indicated by 
a stationary pointer secured to the nonrotating 
supporting structure. Both the bearing dial 
and the pointer are illuminated indirectly from 
below, and a magic-eye indicator is mounted 
beside the pointer. Mechanical stops prevent 
rotation of the shaft through an angle of more 
than 720 degrees to avoid fouling the cable 
from the hydrophone. The through-the-hull in- 
board assembly is shown in Figure 7 with the 
shaft in the raised position. 

Performance 

The through-the-hull equipment performed 
satisfactorily even in moderately rough seas. 
For a large number of trials, bearing accura- 


cies averaged better than ± 2 degrees and 
ranges of over 6,000 yards were obtained on a 
moderately noisy submarine running sub- 
merged at high speed. Listening was found to 
be possible without serious interference at ship 
speeds up to 3 or 4 knots under sail. 

75 SUGGESTED DESIGN 

IMPROVEMENTS 

It is believed that the performance of the 
through-the-hull directive sonic listening equip- 
ment can be improved in several important 
respects. (1) The use of a permanent-magnet 
type of magnetostriction hydrophone would 
eliminate the need for remagnetizing circuits. 
(2) A rubber coupling in the shaft would mini- 
mize the transmission of vibration from the 
hull to the hydrophone. (3) A streamline 
dome over the hydrophone would permit listen- 
ing at speeds greater than 3 or 4 knots under 
sail. (4) Continuous rotation of the hydro- 
phone by means of an electric motor drive 
(with provision for hand training after loca- 
tion of a target) would greatly reduce operator 
fatigue. 


Chapter 8 


AIRCRAFT LISTENING EQUIPMENT- 
TOWED HYDROPHONES 

I 

\ 


INTRODUCTION 

B efore the advent of aircraft listening 
equipment, patrol blimps had little means 
of detecting the presence of submerged sub- 
marines. Prior to the radio sono buoy program, 
as described in the following chapter, the use 
of towed hydrophones for this purpose was 
investigated. It was proposed that submarines 
intercepted on the surface could be tracked 
after submergence by a hydrophone lowered 
from the slowly moving aircraft. 

The two types of towed listening gear de- 
veloped for this purpose were a single direc- 
tional line hydrophone and a pair of nondirec- 
tional hydrophones adapted for binaural 
determination of bearing. Tests of these experi- 
mental models indicated that the designs of the 
hydrophone housings permitted listening at 
water speeds of 20 to 25 knots. However, under 
most conditions the drag resistance of the 
hydrophone cable that was used with both sets 
of gear restricted the operation to lower speeds. 
It was concluded that a radical reduction of the 
cable’s drag-tensile strength ratio, accompanied 
by possible changes in its weight coefficient, 
would be necessary to produce significant im- 
provement in its towing characteristics. Since 
the minimum cruising air speed of the Navy K9 
blimp is 20 to 25 knots, the hydrophones could 
not be used in their experimental form. Offi- 
cial interest was transferred from the towed 
hydrophone program to the more promising 
radio sono buoys with no further improvements 
undertaken by the National Defense Research 
Committee [NDRC]. 

DESIGN CONSIDERATIONS 

The problems encountered in the develop- 
ment of towed underwater listening gear are 
divisible into two parts, electric and mechani- 


cal. From the electrical standpoint, the design 
of transducer units with proper sensitivity and 
directivity characteristics was straight-for- 
ward. The mechanical considerations, however, 
included selection of a streamlined shape for 
the hydrophone housing that would produce no 
turbulence at the speeds anticipated. The trim 
and stability of the housing had to be adjusted 
so that the hydrophone would follow its cable 
without yawing. In the case of the paired 
hydrophones for binaural listening, this stabil- 
ity had to be sufficient to permit accurate de- 
termination of the underwater relative position 
of the hydrophones by adjustment of the two 
cable lengths from the blimp control point. 
Above all, the behavior of towed cable required 
study. Cable drag is so important in towing 
these bodies that, to a first approximation, it 
alone seems to determine the towing load. Both 
the towing depth of the hydrophone and the 
permissible speeds for listening were restricted 
by the towing characteristics of the cable. 

In studying the behavior of cable, a general 
curve was derived which gives the shape of a 
towed cable in air or water for a wide range of 
speeds and loads. This permits the prediction 
of the depth of a towed body from physical 
parameters which can be determined directly. 
Other measurements, made to determine the 
depth of a body as a function of the length of 
submerged cable and the speed, led to an em- 
pirical formula which showed the depth for a 
given length of cable to be inversely propor- 
tional to the square of the towing speed. Be- 
cause of the difficulty of obtaining sufficient 
towing depths at high speeds with simple 
streamlined bodies and cables, diving wings 
and depressors have been used in the design of 
other towed devices.^ 


“ As described in the section on the SR-2 practice 
target. Division 6, Volume 4, Chapter 10. 




67 


68 


AIRCRAFT LISTENING EQUIPMENT TOWED HYDROPHONES 



Figure 1. Blimp-towed hydrophone with diving 

tractor. 

Blimp-Towed Hydrophone 

The blimp-towed hydrophone is an experi- 
mental directional magnetostriction line hydro- 
phone developed by CUDWR-NLL to be towed 
UO feet behind a streamlined diving tractor that 
is itself toived by an aerial cable lowered from 
a patrol blimp. Underivater sounds in the angle 
of the hydrophone' s listening beam can be de- 
tected by the operator aboard the blimp. Stream- 
lined housing permits towing unthout signifi- 
cant turbulence at speeds up to 25 knots. In prac- 
tice, unsatisfactory towing characteristics of 
the cable used ivith this device restrict towing 
to lotver speeds inadequate for general use by 
patrol blimps. 

Construction 

The blimp-towed hydrophone, as shown in 
Figure 1, consists of the hydrophone, cable, and 
diving tractor. The streamlined housing of the 
hydrophone, consisting of a 2-inch cylindrical 
body 4 feet long, with tapered nose and a four- 
wing stabilizing tail, permits towing at speeds 
up to 25 knots with no significant turbulence. 
A weighted nose provides proper trim so that 
the hydrophone travels behind its tractor with 
a fairly stable course, although several tests 
showed a tendency to yaw from side to side. 

The tractor serves to separate the hydro- 
phone from the water noise of the feather 
where the towing cable enters the water. The 
tractor, shaped like the hydrophone housing, 
has a 2-inch cylindrical body, 63 inches long, 
with a four-wing stabilizing tail. A diving vane 
projects on either side from the center of the 
tractor body. A 36-inch towing arm projects 


upward to conduct the towing cable from the 
sloping aerial direction to a horizontal posi- 
tion. A special cable anchor at the top of the 
towing arm distributes the load equally among 
the cable strands to insure towing strength. 

Several types of cable were tested with this 
hydrophone. A tensile strength of 1,500 
pounds was found adequate to carry the towing 
load as well as the additional yawing load. The 
further provisions of electrostatic shielding and 
a waterproofing cover were judged necessary 
for a satisfactory final design. These changes 
would eliminate the introduction of induced 
voltages due to radio interference in the long 
aerial part of the cable and the galvanic inter- 
action of dissimilar metals in sea water. 



Figure 2. Cable reel. 


The cable slip reel that was used to raise and 
lower the hydrophone gear is shown in Figure 
2. It can be adjusted to slip and relieve the 
cable before breaking strength is reached for 
any load between 50 and 2,000 pounds and can 
be operated either by its own electric motor or 
by hand. This slip reel, developed originally for 
towing gliders from airplanes, automatically 
compensates for variable strains on the towing 
structure. 




DESIGN CONSIDERATIONS 


69 


Acoustic Performance 

The construction of the magnetostriction 
line hydrophone provides a directivity pattern 
with main lobes on either side of the line of 
tow. The sharp lobe produced by a single wind- 
ing of a 6-foot hydrophone that was used early 
in the program proved impractical in tests. The 
4-foot hydrophone was wound in three sections 
to provide somewhat broader lobes and was 
sufficiently sensitive to demonstrate the feasibil- 
ity of listening with towed gear of this general 
design. In evaluation tests made by the Navy 
at the Lakehurst Naval Air Station in the sum- 
mer of 1945 a freighter was heard at a dis- 
tance of 5 miles at towing speeds up to 25 
knots. If further development had been car- 
ried out, a conventional baffle would have been 
introduced to eliminate one of the lobes. There 
was also some preliminary discussion of incor- 
porating a relay control to rotate the direction 
of the listening beam, since a hydrophone which 
cannot vary its listening direction relative to 
its course is of limited value. It was felt, how- 
ever, that for real search efficiency a more 
complicated hydrophone circuit would be re- 
quired. 

Towing Performance 

Sufficient tests were made of the blimp-towed 
hydrophone, both from blimps and from sur- 
face vessels, to demonstrate the satisfactory 
design of the housing and the need for radical 
improvement of the towing cable. Whereas lis- 
tening was possible at water speeds up to 25 
knots when the hydrophone was towed from a 
blimp, this top speed was reduced to 10 knots 
in towing from a surface vessel because of the 
necessarily increased length of cable sub- 
merged. Failure of the lightweight cable, under 
the load of yawing and of cable drag, inter- 


rupted several tests. Further study would re- 
quire change of the cable as a first step in im- 
proving the design. Since the K9 blimp cannot 
travel at cruising air speeds less than 25 knots, 
the use of the hydrophone is limited to upwind 
and occasional crosswind courses. 


I 

1 



Figure 3. HW-towed hydrophone. 


HfF-T owed Hyd ro pho ne 

The HW -towed hydrophone is anondirectional 
Rochelle salt hydrophone developed by MIT-USL 
to be towed in pairs from a patrol blimp to per- 
mit binaural detection of underwater sounds. 
Streamlined housing permits toiving ivithout 
observable turbulence at speeds up to 20 knots. 
However, unsatisfactory characteristics of the 
towing cable restricted the toiving to lower 
speeds which were not sufficient for general use 
by patrol blimps. 

Construction 

The HW towed hydrophone, shown in Figure 
3 and schematically in Figure 4, consists of a 
nondirectional crystal transducer housed in a 
streamlined body, attached to a 1 / 2 -inch cable. 
The HW body is 21 inches long and 7 inches in 
diameter, weighing 7.5 pounds in air and 3.3 
pounds in water. By weighing the nose of the 
HW and adjusting the size of the tail fins, the 
trim and stability were adapted for high-speed 
towing. Although the development of the radio 
sono buoys for listening from aircraft led to 



Figure 4. Schematic drawing of HW-towed hydrophone. 



70 


AIRCRAFT LISTENING EQUIPMENT TOWED HYDROPHONES 


withdrawal of official interest from the towed 
hydrophone development, the HW was used in 
laboratory measurement programs, towed from 
surface ships to study ship sounds. 

Inside the HW body, a bank of Rochelle salt 
crystals is housed with a crystal-to-line trans- 
former. The crystals are connected in parallel 
and held in a rigid frame. The pressure faces 
of the bank are coupled to the seawater through 
a film of castor oil and the flexible rubber walls 
of the body. 

Acoustic Performance 

The frequency response of the HW hydro- 
phone, shown down to 1 kc in Figure 5, is flat 
within zb 2 db from 0.15 to 10 kc, measured 
across a matching load. This permits satisfac- 
tory listening at low frequencies. The direc- 
tional pattern of the HW, shown in Figure 6, is 
fairly uniform both in the planes containing 
and the planes perpendicular to the axis. With 
increasing frequency the sensitivity shrinks 
opposite the nose and tail of the HW body. 



In binaural listening the nondirectional fea- 
ture permitted matching the phase of the sepa- 
rate audio signals in a pair of earphones. The 
relative positions of the two hydrophones in the 
water could be adjusted until, by the binaural 
discrimination of the human ear, matching was 
recognized. The auxiliary equipment consisted 
of two identical receiver channels, with equal 
lengths of cable and identical amplifiers, so 
that the two signals were equal in level. With 
this equipment the relative position of a mer- 
chant vessel several miles off could be deter- 
mined. No quantitative measurements were 
made in the tests of binaural listening. 



Figure 6. Directivity patterns of HW-towed hydrophones measured at 3 kc; (A) measured in a plane con- 
taining the axis; (B) measured in a plane perpendicularly bisecting the axis. 


GENERAL CURVE FOR TOWING CABLES 


71 


HW Towing Performance 

The stability of the HW hydrophone as a 
towed body was demonstrated in several tests. 
No turbulence of water along the housing was 
observed at speeds up to 20 knots. It is inter- 
esting to note that the HW reduced the vibra- 
tions of the cable, cutting down the total tow- 
ing load. This was demonstrated in tests of a 



100 300 500 700 900 1100 

LENGTH OF CABLE (L) IN FEET 


Figure 7. Depth-cable length plot for HW-towed 

hydrophones. 

given length of cable with and without an HW. 
No yawing was perceptible from the blimp. A 
pair of HW’s towed in parallel from points 6 
feet apart maintained this separation in the 
water, turning together in parallel courses as 
the blimp changed direction. This stability 
made it possible for the binaural operator in 
the blimp to determine the accurate position of 
the hydrophones by adjustment of the length 
of cable. 

The depth at which the hydrophone travels 
depends upon the trim of the HW housing, the 
towing speed, and . such characteristics of the 
cable as its towing angle, its surface drag as 
function of water speed, and its weight per unit 
length. The trim of the HW housing is adjusted 
by the heavy brass nose so that the hydrophone 
tends to nose downward at low speeds, while 
the stability of the body and tail fin design 
tends to hold it on this course. This downward 
planing effect lets the HW reach considerable 
depths before the upward and downward forces 
become equal. With increasing speed this 
depth decreases, however, and ultimately the 
HW is pulled near the surface where whitecap 
noise interferes with the signal. 


Tests indicate that the depth of the HW for 
a given length of cable is inversely propor- 
tional to the square of the speed. These meas- 
urements were made in preparation for using 
the HW in ship-sound studies, towed behind a 
destroyer. The reinforced cable of the final HW 
design was used, and a syphon-operated depth 
gauge replaced the crystal bank in a standard 
housing. A plot of the empirical relationship 
determined by these tests is shown in Figure 7 
for two speeds. 

The towing load on the cable depends to a 
first approximation upon the length of cable 
that is underwater. Where a great deal of 
cable is submerged, the surface drag, which is 
a function of speed, sets the maximum permis- 
sible towing speed. With 1,000 feet of cable 
underwater this limit was encountered at 14 
knots when the cable covering was seen to fail. 

An estimate of the towing life of the HW is 
based upon tests from a surface vessel where 
failures of one or more cable leads occurred 
near the nose of the hydrophone after 8 hours 
of towing at speeds of 12 to 14 knots. This fail- 
ure repeated itself on 4 successive days, the 
respliced cable always breaking in the same 
place after approximately the same use. This 
implies that the yawing, although impercep- 
tible, was significant in these conditions. The 
use of preformed aircraft cable in place of 
mild steel for the reinforcing strands was pro- 
posed as a remedy. 

« 3 GENERAL CURVE FOR 

TOWING CABLES 

A study was made of the shape of towing 
cable in both water and air to aid in predicting 
the depth of a towed body. A general curve 
was derived^ to give horizontal trail distance 
against depth in dimensionless units. This per- 
mits conversion to any specific case by appli- 
cation of the proper scale factor. The cable 
curve and a plot of its slope are shown in Fig- 
ure 8. The cable curve is the theoretically deter- 
mined curve obtained from the summation of 
forces acting on the cable. The experimental 
points, as indicated in the figure, are derived 
from towing tests with cables in water at speeds 
from 2 to 12 knots and with cable in air at 100 
knots. The agreement with theory is good in all 
cases. 


72 


AIRCRAFT LISTENING EQUIPMENT TOWED HYDROPHONES 


This curve was obtained by letting the depth 
unit Y= (ypd) /T, where y is the depth in feet, 
d the cable diameter in feet, T the tension, and 
p the dynamic pressure. This dynamic pressure 
p^y^RV^ becomes a simple function of the 
speed V if R (Reynolds number) is taken as 
unity. Tests have shown this assumption to be 
valid in the range of speeds encountered in 
towing for the types of cable that were used, 
the horizontal trail distance unit X= (xpd) /T 
was obtained similarly. In using this curve to 
compute the towing depth for a given body at a 
given speed, the observed slope of the cable at 


ing 60 pounds in water towed on 600 feet of 
i/4-inch cable from a blimp. The speed was 
taken as 20 knots. With these quantities, a sub- 
mergence depth of only 2.6 feet is indicated by 
the curve. 

STATUS OF TOWED HYDROPHONE 
DEVELOPMENT 

The satisfactory development of the expend- 
able radio sono buoy [ERSB] led to termina- 
tion of work by the NDRC on towed hydro- 
phones as a means of underwater listening 


°.2.0 


U) 

lo 1.0 


1 

O TOWED UNDERWATER BODY .40" CABLE 2 KNOTS 

— II II II II II 4 " 

^ II II II II II 6 " 

□ II II II II II 8 " 

d' II II 1. .1 I. 10 •• 

p II II II II II 12 '• 

6 BODY TOWED BY AIRPLANE , 100 KNOTS, -f-” 

SMOOTH SASH CORD 
















.iT 

A". 


















>1 









t 













aI 

1 











t 








•s 




1 

1 

1 











1 










1 

1 




X = HORIZONTAL DISTANCE FT 
y = VERTICAL 

s = DISTANCE ALONG CABLE " 
p= DYNAMIC PRESSURE 
'‘"Vsq ft = TRV® 

T= TENSION IN CABLE 

d= CABLE DIAMETER IN FEET 

To= CABLE TENSION AT BODY 

T = To + .008 TT s d p FOR 
SMOOTH CABLE 

B 





y 


T\ DIMtNSIONLESb CUKVt 

OF TOWING CABLES, 
STEADY STATE 

1 

1 



FOR WATER 
OR AIR 

X - 

"T" 

Y - yp** 

^ “ T 

C _ spd 

^ - ~T~ 



1 \ 


o 





1 

1 

1 — V 

1 ^ 
1 


yV 

Tyr - 









1 

1 



1 












1 

1 

1 . 

r- 






1 

1 

SLOPE OF CABLE CURVE | 

1 




— — 




























1 

1 

— 1- 











/ 












A 




































.5 .6 .7 


1 .9 1.0 

xpd 

X=(^) 


1.1 1.2 IJ 1.4 1.5 1.6 1.7 

DIMENSIONLESS TRAIL DISTANCE 


1.8 1.9 2.0 2.1 2.2 2.3 2.4 2.5 


Figure 8. Dimensionless curve for towing cable in steady state. 


its entrance to the water is used to select a 
point A, as shown in the figure where the curve 
has the same slope. Computation of the forces 
acting at the point of attachment of the cable 
to the tractor or towed body determines the 
slope at the foot of the cable, and hence point B. 
The intervening shape of the cable can be taken 
from the curve without further calculations. 

The points A and B shown in Figure 8 cor- 
respond to the case of a streamline body weigh- 


from patrol blimps. At the time neither the 
HW nor the blimp-towed hydrophone was reli- 
able at the cruising speeds necessary for ordi- 
nary blimp operation. The evaluation tests 
made during the summer of 1945 corroborated 
this earlier estimate. Information gained 
about the towing performance of these stream- 
lined bodies and about cable characteristics 
should be found useful if interest in this pro- 
gram is renewed. 


Chapter 9 


AIRCRAFT LISTENING EQUIPMENT — RADIO SONO BUOYS 


INTRODUCTION 

R adio sono buoys are devices designed to 
allow remote detection of sounds in the 
water. These sounds are picked up by a hydro- 
phone and broadcast by radio to a receiving 
station located aboard a ship, in an airplane or 
blimp, or on shore. 

A forerunner of the expendable and direc- 
tional radio sono buoys [ERSB and DRSB] was 
an experimental convoy protection buoy devel- 
oped by the Radio Corporation of America and 
designed to be dropped astern of the convoy. 
Its purpose was to detect trailing submarines 
in time to allow initiation of countermeasures 
against an attack. Although this buoy was not 
carried beyond the experimental stage, it was 
valuable as a basis for the preliminary design 
of the buoys discussed in this chapter. Another 
buoy that contributed to the design of the radio 
sono buoys was the anchored radio sono buoy 
[ARSB], intended for application to harbor 
defense. A brief description is included in 
Chapter 13. 

Before the development of the expendable 
buoys, the magnetic airborne detector [MAD] 
(discussed in Volume 5, Division 6), provided 
the only method for submarine detection from 
aircraft; but its range is short (200 to 300 
feet) and, because it detects wrecks as well as 
submarines, its indications are sometimes mis- 
leading. Therefore, when the tactical situation 
indicated increased use of aircraft in antisub- 
marine patrol, work on an expendable listening 
buoy to supplement MAD and to extend the 
range of detection was inaugurated. 

The first device to be completed and put into 
operational service was the ERSB. Partial evi- 
dence of its effectiveness in the field can be 
gathered from the fact that more than 160,000 
of the units were ordered. The original general 
requirements demanded that the buoy have a 
radio range of at least 5 miles, that it provide 
a continuous operating life of at least 2 hours. 


that it be light in weight! and small in size, and 
that it be suitable for release from blimps by a 
lowering line or other simple means. Decision 
to use the buoy independent of MAD equipment 
and with heavier-than-air craft as well as with 
blimps brought up the additional problem of 
making the buoy suitable for launching from 
airplanes. 

While the underwater sound range of the 
buoy is generally unpredictable because it is 
dependent on water conditions and on the ma- 
neuvers and evasive tactics of the target, sub- 
marine detection ranges of 2 miles or more 
may be expected under excellent conditions. 
An aircraft at an altitude of 5,000 feet has an 
effective maximum radio range of 35 miles. 

With the ERSB, a submerged submarine can 
be kept under aural observation, oil slicks can 
be investigated, and damage to a submarine 
during attack can be more readily ascertained. 
It is also possible to track a moving submarine. 
For this function it is necessary to employ 
several buoys simultaneously — to lay them in 
a pattern and note the relative intensity and 
variation in intensity of the sound at the differ- 
ent buoys. 

It was felt, however, that these functions 
could be more effectively performed if a buoy 
could be developed which would be capable of 
broadcasting not only the underwater sounds it 
might pick up but also the direction from which 
these sounds come. Tactical operations would 
be expedited, observation time would be re- 
duced, and space in aircraft would be saved. 

The idea of a directional buoy was conceived 
early in the radio sono buoy program. There- 
fore, when the major engineering problems of 
the ERSB had been answered, work on the 
DRSB was started. A directional hydrophone 
was substituted for the nondirectional ERSB 
hydrophone. This hydrophone rotates continu- 
ously to provide 360-degree scanning in a hori- 
zontal plane around the buoy. To provide bear- 
ing information, the frequency of the radio 


73 


74 


AIRCRAFT LISTENING EQUIPMENT RADIO SONO BUOYS 


transmitter varies with rotation of the buoy; 
the receiver is capable of interpreting this fre- 
quency variation in terms of bearing. 

To accomplish these ends, it was necessary 
to redesign the several electronic circuits, to 
select a hydrophone with suitable characteris- 
tics, to devise a method for supporting rigidly 
and for rotating the hydrophone, and to develop 
a scheme for providing a reference direction to 
which rotation could be related. Furthermore, 
to obtain adequate directivity, a larger hydro- 
phone than that in the ERSB was needed, and 
this, together with the mechanism needed to 
rotate the buoy, added appreciably to its weight 












1 

1 


Figure 1. The expendable radio sono buoy [ERSB]. 


and bulk. Therefore a general redesign of the 
buoy was required. 

Although emphasis in the chapter is placed 
on the use of the ERSB and DRSB from air- 
craft, they were often launched from ships. The 
directional buoy was particularly well suited 
for this use because ship noise did not, gener- 
ally, interfere with target-noise detection. 
Whether operated from aircraft or ships, both 
buoys played a major part in the listening pro- 
gram. The discussions in this chapter cover 
their uses, operation, performance, and reasons 
for their development along with suggestions 
for further development of this type of listen- 
ing equipment. 

Expendable Radio Sono Buoy [ERSB] 

The ERSB, also designated by the Army- 
Navy type number AN /CRT-1 A, is a device 
designed to be dropped from airplanes or blimps 
by means of a small, self-contained parachute 
and used to pick up the underwater sounds of 
submarines and transmit them to the aircraft 
by radio. It is made up of a cylindrical magneto- 
striction toroidally wound hydrophone and an 
amplifier connected to an f-m radio transmitter. 
The sonic hydrophone, amplifier, and transmit- 
ter, along ivith a battery power supply, are in- 
corporated in a ivaterproofed cardboard tube 
about 30 inches long and U inches in diameter 
iveighing approximately 12 pounds. The trans- 
mitter and batteries are housed in an upper 
compartment, separated by a watertight bulk- 
head from the hydrophone, cable, and release 
mechanism in the lower compartment. The 
transmitter operates on frequencies between 67 
me and 72 me and has a maximum range of 
about 35 miles when the aircraft is at an alti- 
tude of 5,000 feet. The device has an operating 
life of 2 hours to hours after planting, after 
which a Carbotvax plug dissolves and permits 
the buoy to sink. In order to track a moving 
submarine, several buoys may be dropped in a 
pattern surrounding the known or suspected 
location of the submarine. A receiver designated 
by the Army-Navy type number AN/ARR-3 is 
carried in the aircraft. This receiver has up to 
12 channels corresponding to the particular fre- 
quencies of a like number of buoys. A high de- 
gree of automatic frequency control [AFC] is 



EXPERIMENTAL WORK 


75 



Figure 3. Mark II (right) and Mark III experi- 
mental transmitters with first experimental buoy 
(center) . 

tion reasonably well but lacked sufficient a-f 
amplification. 

Mark II. A redesigned transmitter employed 
five tubes mounted in a multiple-deck chassis 
with the r-f and audio circuits separated. This 
transmitter, together with a special magneto- 
striction hydrophone, was housed in a water- 
proof paper tube and provided with an antenna 
rod to make up the first complete unit, desig- 
nated the Mark II buoy. In this model, shown 
in Figure 3, the hydrophone and its connecting 
cable were held in the lower portion of the 
housing by paper tape designed to break upon 
impact with the water and allow the hydro- 
phone to drop to the limit of the cable. Four 
of these units were tested in free-fall drops 


incorporated to compensate for any lack of fre- 
quency stability in the buoys. This equipment 
was developed by CUDWR-NLL. The Emerson 
Radio and Phonograph Company and the Gen- 
eral Textile Mills fuimished the manufacturing 
designs and the parachutes respectively. 

’2 EXPERIMENTAL WORK 

Early Models 

The basic requisite of an aircraft-launched 
buoy was considered to be a small, rugged, 
lightweight radio transmitter having low power 
consumption. After settling on the desirability 
of f-m transmission, a number of experimental 
models were constructed in rapid succession. 

Mark I. The first transmitter model, Mark I, 
shown in Figure 2, was of single-deck construc- 


Figure 2. The Mark I experimental transmitter 

tion and employed four battery-supplied 
vacuum tubes. This design met the require- 
ments as to size, weight, and power consump- 



76 


AIRCRAFT LISTENING EQUIPMENT RADIO SONG BUOYS 


from a blimp of 100 and 200 feet, and although 
three of the four operated after reaching the 
water, their transmission was characterized by 
considerable carrier-frequency shift. Subse- 
quent examination showed that some vacuum- 
tube elements had been bent by the shock of 
impact. These tests indicated the necessity for 
some means of decreasing the impact and insur- 
ing vertical entry of the buoy into the water. 

Mark III. In an effort to solve the launching 
problem, a Mark II buoy was lowered from a 


diameter ratio necessary for stability in the 
water. 

Mark IV. The final experimental model, 
Mark IV, employed a multiple-deck type of 
construction with the antenna coupling, radio- 
frequency, and audio-frequency circuits sepa- 
rated and arranged in order from top to bot- 
tom. In this buoy the upper portion of the 
housing was covered on the outside with a 
copper foil sheet which served as an electro- 
static shield. The parachute, attached to a ring 



Figure 4. Prototype (Mark II) receiver 


blimp by a handline. The handline method 
proved so troublesome that it was promptly 
abandoned. A third design, Mark III, was 
equipped with a small parachute and was 
dropped from an altitude of 500 feet, suffering 
no damage. On the basis of these tests, this 
design was considered a satisfactory solution 
to the problem of launching the buoys from 
lighter-than-air craft. The Mark III design 
was abandoned, however, because its reduction 
in length was not accompanied by a corre- 
sponding reduction in diameter, and thus the 
buoy-housing tube did not have the length- 


at the top of the antenna, disengaged automati- 
cally when the buoy came to rest in the water. 
Successful tests of the Mark IV resulted in 
development of a model more adaptable to pro- 
duction, the Mark IV B, of which small lots 
were ordered for further tests. 

Mark IV C. The development work up to this 
time had been directed solely toward produc- 
tion of a buoy which could be launched from 
blimps, although it had early been realized 
that launching from airplanes might eventually 
be desired. For this reason, test launchings 
from airplanes were made to determine the 





EXPERIMENTAL WORK 


77 


feasibility of adapting the Mark IV B model 
for use from heavier-than-air craft. These 
tests indicated the need for an improved para- 
chute as well as certain other mechanical 
changes to make the unit suitable for launch- 
ing at higher speeds. Further tests, using a 
redesigned parachute arrangement with the 
shrouds attached directly to the buoy top 
showed that a nearly vertical fall of the buoy, 
and thus better placement accuracy, could be 
attained. This model, with the redesigned para- 
chute, was designated the Mark IV C. It was 
considered satisfactory for launching from all 
types of aircraft and orders were placed with 
a manufacturer for a preproduction lot of these 
units for operational and service tests in the 
field. 

Mark IV D. Further improvements, directed 
largely toward facilitating storing and han- 
dling of the buoy, were incorporated in a still 
later model, the Mark IV D. The changes in- 
cluded the provision of a permanently mounted 
telescoping antenna, a compact parachute pack, 
an improved battery switch, and minor altera- 
tions in the a-f and r-f circuits aimed at in- 
creasing the uniformity of transmission char- 
acteristics. A preproduction lot of this unit 
was also ordered. 

Production Model (AN/CRT-1) Buoy 

Extensive service tests of the preproduction 
buoy models indicated that the development 
had progressed to a stage which justified quan- 
tity production to meet pressing needs. Speci- 
fications were drawn for a Mark IV E design 
incorporating a compact battery assembly 
using standard cells, a further improved hydro- 
phone release mechanism, and a new, rigid, 
humidity-resistant and fungus-proof parachute 
pack. These specifications were used in manu- 
facturing, first for the Army and then for the 
Navy, substantial production quantities of the 
Mark IV E buoy later designated by the Ser- 
vices as the AN/CRT-1 unit. 

Mark II (Prototype) Model Receiver 

The receiver used in the early buoy tests was 
of conventional FM design in most respects, 
but of small size and weight and with a wide- 


range AFC system capable of compensating 
for carrier frequency shifts of as much as 
±200 kc. On the basis of tests in which three- 
buoy patterns were used, this receiver was 
designed to accommodate three frequency chan- 
nels, but later tests indicated that patterns of 
four buoys were sometimes more effective. For 
this reason, and in anticipation of even more 
extensive future requirements, the receiver was 
redesigned to provide for six channels. The 
new receiver design, known as Mark II and 
shown in Figure 4, served as a prototype for 
the Service-designated AN/ARR-3 receiver 
which was put in production. 



Figure 5. Type AN/CRT-1 and AN /CRT-1 A buoys. 





78 


AIRCRAFT LISTENING EQUIPMENT RADIO SONO BUOYS 


9 3 FINAL PRODUCTION DESIGN 

^ AN/CRT-IA Buoy 

With the AN/CRT-1 buoy and the AN/ARR- 
3 receiver in substantial quantity production 
approximately 9 months after the start of the 
development, further, more deliberate work was 
directed toward redesigning the buoy for im- 
proved performance and greater production 
economy. Extensive service use of the earlier 
buoy models indicated the need for redesign 
work to eliminate vacuum-tube microphonics. 
The new unit, evolved as a result of this de- 
velopment, had improved electric characteris- 
tics, including greater sensitivity and the elimi- 
nation of microphonics. It also achieved the 
objectives of reduced weight, size, cost, and 
greater ease of production. This buoy became 
the prototype for the final production model, 
type AN/CRT-IA, which was manufactured 
in large quantities. The AN/CRT-1 and AN/ 
CRT-IA buoys are compared in Figure 5. 

Housing 

The buoy housing consists of a convolute- 
wound Kraftboard tube, approximately 30 
inches long by 4 % inches in diameter, sepa- 
rated by a watertight bulkhead into an upper 
compartment for transmitter and batteries and 
a lower compartment for the hydrophone, 
cable, and release mechanism. The cutaway 
picture in Figure 6 shows this construction. 
A wooden cap, fitted with a rubber gasket and 
clamping screws, seals the top of the tube and 
serves as a mounting for the antenna and para- 
chute assembly. This cap contains a soluble 
Carbowax"" plug to sink the buoy for security 
after several hours' operation. Four %-iuch 
holes are cut through the tube wall at the upper 
end of the lower compartment to insure hood- 
ing and to provide a cushioning effect by regu- 
lating air release as the buoy strikes the water. 
The upper part of the tube is sprayed on the 
outside with atomic copper to form an electro- 
static shield; the whole tube is coated inside 
and out with a water-resistant compound. 

» Carbowax is the trade name of a hard, opaque, 
nonhygroscopic polyethylene oxide solid having a melt- 
ing point above 135 F. 



a^TOP cap 

GASKET 


BATTERIES 


-BULKHEAD 


TRANSMITTER WITH 
TRANSPARENT 
PROTECTIVE 
SHIELD 



HYDROPHONE 


HYDROPHONE 

RELEASE 


Figure 6. Section view of AN/CRT-IA buoy. 


FINAL PRODUCTION DESIGN 


79 


Hydrophone Release Mechanism 

The bottom end of the housing terminates 
in a cast metal ring which aids in stabilizing 
the buoy in the water and provides a mount- 
ing for the hydrophone release mechanism. 



Figure 7. Hydrophone release, triggered as at 
moment of impact with sea. 


This mechanism, illustrated in Figure 7, con- 
sists of a spring arrangement which holds the 
hydrophone firmly in place during shipping 
and handling but which automatically triggers 


on impact with the water and permits the hy- 
drophone to drop to the limit of its 24-foot 
cable. 

Hydrophone 

The hydrophone, shown in Figure 8, is a 
cylindrical magnetostricjtion unit, toroidally 
wound directly on a nickel shell. It represents 
a new design first applied to sono buoy use in 
the AN/CRT-IA model. This construction per- 
mits storing of the cable inside the hollow shell 
and effects a reduction in length of nearly 4 
inches compared with the more conventional 
magnetostriction unit used in earlier models. 
In addition the hydrophone yields greater ef- 
fective voltage at the input of the first tube 
and has more uniform operating characteris- 
tics. The family of frequency-response charac- 
teristics, shown in Figure 9, rises sharply with 
increasing frequency, complementing the re- 
verse type of characteristic typical of sub- 
marine sounds and inherent sea noise, and 
hence providing a substantially flat overall 
modulation voltage. 

Transmitter 

The FM transmitter utilizes five vacuum 
tubes providing approximately 90 db of audio- 
voltage gain and an effective r-f antenna radia- 
ti(fn of about 0.1 watt. Frequency modulation 
was used in preference to amplitude modulation 
for three main reasons: (1) Its signal-to-noise 
ratio was considered of vital importance be- 
cause the receivers are always used in close 
proximity to aircraft engines, with the atten- 
dant possibility of ignition interference; (2) 
it provides precise automatic control of volume 
of all signals sufficiently strong to fall within 
the effective operating range of the receiver; 
and (3) it eliminates interference between two 
buoys of the same color frequency. This applies 
when extra buoys are dropped, in tracking, 
before the original buoys have ceased operat- 
ing. 

The AN/CRT-IA, in contrast to the earlier 
buoy transmitters with multiple-deck type con- 
struction, employs a single, rectangular plate, 
mounted vertically, with the audio amplifier 
and the reactance tube on one side, and the r-f 


80 


AIRCRAFT LISTENING EQUIPMENT RADIO SONG BUOYS 


circuit on the other. Compactness and improved 
isolation between the a-f and r-f circuits are 
thus achieved, as indicated in Figure 10. Free- 
dom from microphonic noise is achieved by 
use of four shockproof rubber mountings for 


be encountered in shipment, impact, or use, 
and the whole transmitter assembly is enclosed 
in a transparent acetate tube for protection 
when withdrawing the unit from the housing 
for installation of batteries. 



Figure 8. Toroidally-wound CRT-1 A hydrophone. Plastic inner structure protects cable connection and pro- 
vides means for supporting the hydrophone within the buoy. 


the chassis plate as well as separate rubber 
mounting for each tube socket. This eliminates 
the necessity for the undependable and expen- 


o 



Figure 9. Frequency response of fifteen CRT-IA 
hydrophones. 


sive process of tube selection. The vacuum 
tubes are provided with locking-type shields to 
prevent their being loosened by the jarring to 


A schematic circuit diagram of the AN/CRT- 
lA transmitter is given in Figure 11. By sub- 
stituting pentodes for triodes, a change was 
made from three a-f amplifier stages to two, 
with resultant decrease in the number of cir- 
cuit components over the earlier sono buoy 
designs. Use of permeability tuning in three 
of the four r-f tank circuits simplified the pro- 
duction process because the type of adjustment 
required is less critical than that necessary 
with the variable capacitors that had been 
used formerly. This type of tuning is also less 
sensitive to the shock of impact or rough han- 
dling and so provides increased frequency sta- 
bility. 

Battery Supply 

The battery assembly consists of four stand- 
ard 1.5-volt flashlight cells in parallel for the 
filaments, and two series-connected 67.5-volt 
miniature B batteries for plate voltage. Suffi- 
cient battery capacity is available for a con- 
tinuous operating life of approximately 4 hours. 


FINAL PRODUCTION DESIGN 


81 


Antenna 

The antenna is a 39-inch telescoping quarter- 
wave tube mounted on the buoy housing cap 
with 9^^ inches of its base enclosed in a water- 
tight insulating sleeve to avoid short-circuiting 
by waves. It is coupled to the r-f amplifier tube 
by a tuned circuit which matches the imped- 


Parachute 

The parachute, 24 inches in diameter, is of 
muslin, dyed orange for increased visibility. Its 
shroud lines are attached to the buoy cap. Dur- 
ing shipping and storage it is contained in a 
moisture-resistant pack fitted around the insu- 
lating sleeve at the base of the antenna. After 



Figure 10. R-F (left) and A-F sides of CRT-IA transmitter. 


ances of the antenna and transmitter and helps 
to stabilize operation. It accomplishes the lat- 
ter by isolating the tuned transmitter circuits 
from the direct influence of any variations in 
antenna characteristics due to motion of the 
buoy. 


launching, the pack cover is torn loose by a 
static line attached to the plane, the parachute 
blossoms around the extended antenna which 
protrudes through a 3-inch hole in the top (Fig- 
ure 12), and the pull on one of the shrouds 
withdraws a switch pin and so turns on the 



82 


AIRCRAFT LISTENING EQUIPMENT RADIO SONO BUOYS 


transmitter. As the buoy reaches the surface 
after its initial plunge into the water, the para- 
chute settles around the antenna base. 

AN/ARR-3 Receiver 

The AN/ARR-3 buoy receiver is a 13-tube 
superheterodyne type, shown in Figure 13 and 
schematically in Figure 14. It provides for the 
reception of frequency-modulated signals in 


This AFC feature is necessary since the trans- 
mitter carrier frequency goes on the air from 
a cold start and its stability is likely to be 
affected by such factors as decreasing battery 
voltage, severe mechanical shock, and sudden 
temperature changes. A feature of the AFC 
which was beneficial in the field was the ease of 
tuning it gave, since the pattern technique re- 
quires continual switching from one buoy to 
another. 


REACT. TUBE 



any of six channels covering the range 67.2 to 
72.2 me, and incorporates an automatic fre- 
quency control [AFC] system capable of shift- 
ing the local oscillator sufficiently to compen- 
sate for carrier-frequency drift up to ± 200 kc. 


The AFC is accomplished by means of a con- 
trol voltage which is developed in the discrimi- 
nator stage and fed back to the control grid of 
the phase amplifier tube. This tube, in turn, 
through the oscillator-control tube, regulates 




FINAL PRODUCTION DESIGN 


83 



Figure 12. ERSB during launching. 


change as necessary to compensate for the 
amount of carrier-frequency shift. The delayed- 
action time constant of this circuit is sufficient 
to avoid interference with the reception of sig- 
nals that deviate zt 75 kc even when the varia- 
tion is at as low a frequency as 50 c. When it 
is desirable for purposes of checking the buoy 
transmitter frequencies, the AFC feature can 
be cut out by means of a switch on the receiver 
panel. 

Additional operating characteristics of the 
receiver include a measured sensitivity of from 
8 to 12 fiv for 20 db quieting, a minimum image 
rejection of 49.6 db, and an a-f response flat 
within 5 db between 100 and 10,000 c. Each 
unit is provided with a separate dynamotor 
power supply which uses a 24-volt source. 

In other respects, the circuit of the AN/ARR- 
3 receiver follows conventional FM design. It 
utilizes two r-f stages, the first untuned (trim- 
mer-adjusted at the time of installing the re- 
ceiver), the second tuned by one section of the 
three-gang condenser, operated by the tuning 
control, whose other two sections tune the oscil- 
lator and mixer stages. The i-f amplifier, with 



Figure 13. Type AN/ARR-3 receiver and power supply. 


the amount of out-of-phase current or reactive 
load that is reflected across the tank circuit of 
the local oscillator and causes its frequency to 


a pass band of 150 kc centered at 5,000 kc, con- 
sists of three stages, one of which is a limiter. 
This is followed by a discriminator and a two- 







84 


AIRCRAFT LISTENING EQUIPMENT RADIO SONG BUOYS 



Figure 14. Schematic circuit diagram of AIlR-3 receiver. 


stage a-f amplifier. Outputs of 300- and 5,000- 
ohm impedance are provided to accommodate 
electromagnetic or crystal type headsets. 

Sections of the tuning-control dial, cor- 
responding to the frequency bands of the op- 


erating channels of the buoys, are colored to 
match the frequency-identification colors on the 
individual buoys and thus assist the operator in 
identifying the particular buoy to which he is 
listening. 










OPERATION OF EQUIPMENT 


85 



Figure 15. ERSB before launching: (A) Withdrawing static line removes soluble seal protector plug. 
(B) A 1-0 seal and cardboard covers ripped and removed. Static line is then tied to airplane. 


OPERATION OF EQUIPMENT 

Testing 

At operational bases each buoy is given a 
performance test in which the antenna current 
is measured or the actual radio signal is 
checked with a field-strength meter. In addi- 
tion, the frequency adjustment of the trans- 
mitter and the performance of the hydrophone 
and audio system are checked by means of a 
calibrated AN/ARR-3 receiver using the buoy 
hydrophone as a microphone to pick up speech 
or tapping sounds. Similar tests are repeated 
just before the buoys are placed in aircraft for 
use and, if possible, a quick listening check is 
made just before launching. 

Launching Methods 

The buoys are commonly launched in one of 
three ways, by hand through an open hatch, by 
means of a built-in launching tube, or auto- 
matically from the bomb bay. In hand launch- 
ing, the buoy is first prepared by removing the 
protective packing and extending the antenna. 
Next, a few feet of static line are pulled out. 
This operation withdraws the rubber protector 


from the hole containing the soluble plug. Then 
the moisture-resistant and outer cardboard 
parachute covers are ripped off. These pro- 
cedures are illustrated in Figure 15. The end 
of the static line is then secured to the plane, 
and the buoy, with its antenna pointing down 
and aft, is thrust out through the hatch. After 
a 20-foot free fall which pays out the balance 
of the static line, the end of the line rips the 
last paper cover from the parachute, permit- 
ting the chute to open, and a weak link parts, 
leaving the line attached to the plane. The blos- 
soming parachute causes one of the shrouds to 
actuate the switch which sets the buoy trans- 
mitter in operation. 

In planes equipped with a launching tube, the 
sequence of operations is the same as in hand 
launching except that the buoy is ejected by an 
elastic cord when a holding device is triggered. 
In bomb-bay launching, the buoys are attached 
to the bomb shackles by means of metal straps, 
and the entire launching procedure, except for 
extending the antennas and tying the static 
lines before placing the buoys in the racks, is 
automatic. This method has the advantage of 
permitting several buoys to be kept in constant 
readiness for dropping in quick succession if 




86 


AIRCRAFT LISTENING EQUIPMENT RADIO SONO BUOYS 


desired and permits attachment of dye or other 
markers directly to the buoys. 

Slicks and Markers 

Since the buoys are not easily seen from the 
air, several methods of marking their location 
have been developed. In the daytime, metallic 
or dye slicks and float lights, giving off both 
smoke and flame, are used. Since the metallic 
bronze and aluminum slicks have generally too 
short persistence except in very calm seas, and 
the Mark V float light'" lasts only about 15 
minutes, fluorescein and rhodamine dyes were 
favored. From an altitude of 3,000 feet, fluo- 
rescein is visible from at least 10 miles. Al- 
though rhodamine is visible from only about 5 



Figure 16. One type of pattern employed in buoy 
operation. 


miles, its visibility is good on dull days. At 
night, the Mark V float light was found to be 
the best available means of marking the buoy 
locations. 

Search Procedure 

The buoys are used either singly, to investi- 
gate irregularities such as oil slicks, disap- 
pearing radar blips, and MAD indications or 


^ Discussed in Division 6, Volume 18. 


in patterns for tracking submarines. In the 
latter application, five buoys are usually laid in 
a square 2 miles on a side, with one unit at the 
center marking the best estimate of the sub- 
marine’s location. By successive switching 
among the five buoys, the path of the submarine 
can be traced through changes in the relative 
intensity of the sound from the different loca- 
tions. If the submarine passes out of the pat- 
tern, a new square is started, as indicated in 
Figure 16, by dropping three more buoys, 
using as a center the one from which the most 
intense signal was obtained. Since these tactics 
may involve use of more than six buoys, a plan 
for increasing the number of channels to 12 
has been considered and implemented as a 
means of preventing confusion which might 
arise in spite of the ability of FM circuits to 
select the strongest signal. 


^5 SUGGESTED IMPROVEMENTS 

In large-scale use, the ERSB has proved to 
be an effective aid to antisubmarine warfare. 
Official reports indicate that the information 
supplied by these buoys has been largely re- 
sponsible for establishing and maintaining con- 
tacts that have led to the destruction of enemy 
submarines on many occasions. It is believed, 
however, that a number of improvements would 
make the device even more effective. Foremost 
among these is the provision of directional in- 
dications to permit much more accurate loca- 
tion of submerged targets, longer listening 
ranges, and the use of fewer buoys. 

In addition to the directional feature, it is 
considered desirable to double the number of 
frequency channels for the nondirectional 
buoys, to provide for connecting the receiver 
output directly into the plane’s intercommuni- 
cation system, and to develop containers for 
marker dyes which will dispense the dyes on 
impact with the water but which will not break 
in shipping or normal handling. The means of 
providing for the extra frequency channels and 
connection to the intercommunication system 
have received attention since the AN/CRT-1 A 
buoy and AN/ARR-3 receiver were put into 
production. These modifications have been in- 
corporated in the subsequent production models. 



PRELIMINARY DEVELOPMENT 


87 



Figure 17. The directional radio sono buoy as it 
appears in the water. 


Directional Radio Sono Buoy [DRSB] 

The DRSB is a buoy that may be dropped 
from aircraft by means of a small parachute and 
is used, like the ERSB, to pick up the under- 
ivater sounds of submarines and transmit the 
sounds to the aircraft by radio, at the same time 
indicating the direction from ivhich the sounds 
are arriving. It consists of a directional sonic 
listening hydrophone, sonic amplifier, rotating 
mechanism, and an f-m radio transmitter. The 
ivhole system is incorporated in a tubular buoy 
about 52 inches long, 6 inches in diameter, and 
weighing approximately 30 pounds. The con- 
tainer is composed of two tubes attached end to 
end. The upper one, a watertight buoyancy 
chamber, contains the radio chassis and bat- 
teries; the loiver holds the folded hydrophone 
and the motor mechanism. Transmission of di- 
rectional information is accomplished by means 
of a compass-capacitor ivhich causes the fre- 
quency of the radio transmitter to vary with 
rotation of the buoy. A receiver, carried in the 
aircraft, is provided with a circuit to translate 
changes in the transmitting frequency into di- 
rectional indications, shoivn on a meter. These 
buoys were developed by CUDWR-NLL. 


PRELIMINARY DEVELOPMENT 

It was early envisioned that the DRSB should 
employ a unidirectional hydrophone which 
would be rotated continuously and thus provide 
360-degree scanning. To provide bearing in- 
formation it was planned to vary the FM 
transmitter’s carrier frequency with rotation 
of the buoy by connecting into the tank circuit 
of the oscillator a variable condenser so ar- 
ranged that one plate would be held stationary 
while the buoy rotated. At the receiver end, the 
drift of carrier frequency would be indicated 
on the face of a zero-center voltmeter so con- 
nected as to show variations in the voltage ap- 
plied to the automatic frequency control section 
of the receiver as the carrier varied from its 
prescribed frequency. The detailed methods of 
accomplishing these functions required con- 
siderable experimentation. 

Selection of a Hydrophone 

In selecting a suitable hydrophone it was 
necessary to consider both its acoustic charac- 
teristics and its general mechanical adapta- 
bility. 

Extensive tests resulted in agreement among 
observers that a 2-foot straight magnetostric- 
tion-type hydrophone was definitely superior to 
all others tested.^ Because it provided the best 
bearing accuracy and range, it was adopted 
despite its somewhat greater size and weight. 

Hydrophone Support 

The support for the hydrophone and motor 
must be torsionally rigid and noise-free. After 
trying and discarding various types of flexible 
shafting and cable, it was found that tele- 
scoping metal tubing could be designed to meet 
the requirements. Although this type of coup- 
ling limits the practical depth at which a hydro- 
phone can be suspended, it was used on all 
DRSB models. 

Orientation Reference System 

Two means were investigated for providing 
a reference direction to which hydrophone 
bearing could be related. One of these made use 


88 


AIRCRAFT LISTENING EQUIPMENT RADIO SONO BUOYS 


of the direction of the wind. A frequency- 
deviating capacitor was coupled to a wind vane 
mounted on the exposed portion of the antenna. 
This method worked but was erratic in a puffy 
wind. For this reason and because of the neces- 
sity for knowing wind direction at the airplane, 
this scheme was abandoned. 

The second method involved the use of a 
liquid compass to give bearings with respect to 
magnetic north. The compass card was re- 
placed by an eccentric metal disk which, to- 
gether with a fork straddling the disk, com- 
prised the frequency-deviating capacitor. The 
orientation of the metal disk was held constant 
by action of the earth’s magnetic field. This 
compass-capacitor proved to be quite reliable 
and was incorporated in all models of the 
DRSB. 

Methods of Rotating the Hydrophone 

A number of methods for rotating the hydro- 
phone were investigated. Those involving the 
use of wind action, wave action, electric power, 
compressed air, and coiled springs were con- 
sidered and abandoned. A fairly satisfactory 
rubber-band motor^ was constructed and actu- 
ally used in some of the early experiments with 
dummy buoys. However, a gravity-type motor 
proved to be most suitable. This consisted of a 
spool of cord attached to the buoy and a weight 
arranged so that as the weight fell away from 
the buoy the spool of cord would unwind and 
rotate the buoy and hydrophone with respect to 
a set of water reaction blades. The weight falls 
approximately 100 feet for each hour of buoy 
operation. All complete DRSB models used this 
type of motor. 

DEVELOPMENTAL MODELS 
Mark I Buoy 

The first DRSB model to be launched from 
aircraft was designated Mark 1. It was housed 
in a paper tube 6 1/2 inches in diameter by 38 V 2 
inches long, and weighed 22 pounds complete. 
A parachute eased its drop and guided it ver- 
tically into the water. Upon impact with the 
water, the antenna erected, the parachute cast 
off, and the hydrophone and motor dropped to 
the end of the telescoping support tube. 


Figure 18 shows the top assembly removed 
from its housing to disclose the shock-mounted 
transmitter, battery supply, and compass-ca- 
pacitor. Above the top deck is the parachute 
compartment. The transmitter was a non- 



PARACHUTE 

COMPARTMENT 


COMPASS 


BATTERIES 


TOP DECK 


TRANSMITTER 


Figure 18. Top section, removed from housing — 

Mark I buoy. 

magnetic modification of the AN/CRT-IA f-m 
transmitter described earlier in this chapter 
in connection with the ERSB. The compass- 
capacitor was of the double-pivot type, devel- 
oped for the purpose and notable for its com- 
pact size. The antenna consisted of a set of 
telescoping metal tubes which collapsed into 
the transmitter chassis. Impact with the water 
actuated a spring release which erected the an- 
tenna and cast off the parachute. 

The bottom end assembly with components 
folded to fit into the buoy housing is shown in 
Figure 19A. The two 1-foot sections of the 
hydrophone appear folded upwards, one sec- 
tion on each side of the telescoped support. 


DEVELOPMENTAL MODELS 


89 


Below are the two flexible reaction paddles 
coiled around the gravity motor. On striking 
the water this unit drops out of the buoy 
housing, the telescoped metal tubing extends 


In efforts to improve antenna behavior, a 
spiral of springy metal ribbon was tried as the 
antenna. Extended, it took the form of a 
slightly tapering tube; compressed, it resem- 



Figure 19. Bottom assembly of Mark I buoy. (A) folded; (B) operating position. 


to its full length, and the hydrophone and 
paddle blades unfold into operating position as 
shown in Figure 19B. The metal base plate of 
the buoy serves as the weight for the gravity 
motor. 

Tests on Mark I revealed the need for further 
improvements, especially in mechanical design 
of some of the components. Rough water caused 
erratic action of the small compass; even in 
calm water, bearing accuracy was only about 
20 degrees. The hydrophone release and the 
antenna-parachute release mechanisms did not 
always function. Trouble was also encountered 
in the proper functioning of the parachute. 


bled a clock spring in shape. However, the re- 
lease mechanism proved to be undependable. 

Mark II Buoy 

The antenna used in the Mark II buoy was a 
slightly concave strip of spring steel, similar 
to that employed in measuring tapes, which 
maintains itself stiffly straight as it is unreeled. 
In the assembled buoy this strip was bent over 
against the side of the buoy and held in this 
position by a removable cap which also served 
to house the parachute. This scheme not only 
assured that the antenna would immediately 




90 


AIRCRAFT LISTENING EQUIPMENT RADIO SONO BUOYS 


erect but also provided a more certain means 
of removing the parachute safely. In the 
former design, withdrawal of the parachute 
was complicated if the buoy was not properly 
oriented with respect to the static line. Fur- 
thermore, removal of the parachute cap de- 
creased the top weight of the buoy in the water, 
thereby improving stability. 

The gravity motor of the Mark II buoy em- 
ployed stiff paddle blades, hinged to fold within 
the buoy housing. An alternate design achieved 
a slight reduction in overall length by arrang- 



Figure 20. Slave-master-type compass capacitor 
developed for Mark II buoy. 

ing for the motor to slide up into the folded 
assembly when closed. This advantage did not 
appear to justify the complications introduced. 

A new and larger compass-capacitor, utilizing 
the slave-master principle, was designed to 
insure smoother operation in rough water. A 
drawing of the unit is shown in Figure 20. The 
master disk was mounted on a single pivot to 
permit it to remain horizontal despite tilting 
of the compass case by as much as 25 degrees. 
This disk, in turn, magnetically drove the small 
doubly pivoted slave disk, which was the eccen- 
tric plate of the frequency-deviating capacitor. 

No formal tests were made of the Mark II 
buoy but overside tests demonstrated the 
marked advantage in bearing accuracy offered 
by the larger slave-master type of compass. 


Mark III Buoy 

This model was a parallel development of the 
Mark II buoy and incorporated most of the 
features and functions of that unit. It differed 
primarily in the use of the smaller Mark I 
compass. 

Drop tests of this buoy demonstrated the 
advantages of the separable parachute cap and 
the simple spring-steel type antenna. However, 
on listening, there were unwanted noises, at- 
tributed to the torque tube and paddle. Fur- 
thermore, bearing accuracy was not good in 
rough water. This unreliable behavior of the 
compass persisted even after the Mark I com- 
pass was replaced by a small one of the slave- 
master type. The large surface area of the flat- 
spring antenna caused it to buckle occasionally 
in the wind. Moreover, the antenna was 
mounted off center to decrease sharpness of 
bend when folded, and as a result even a 
moderate wind interfered with smooth rotation 
of the buoy. 

Mark IV Buoy 

The Mark IV buoy was built around the 
larger more stable slave-master compass of 
the Mark II unit. The antenna was moved to 
the center of the top deck, and a rod was sub-' 
stituted for the major portion of the antenna. 
Three thicknesses of spring-steel tape were 
used for the bottom section of the antenna to 
allow it to be bent down against the side of the 
buoy and at the same time provide greater 
rigidity. 

The keying grooves in the succeeding sections 
of the telescoping support tube were made with 
graduated radii so they would nest together 
more precisely. In addition, small cylindrical 
sleeves were introduced between adjacent sec- 
tions so that, when extended, the tube would be 
more rigid and noise-free. 

Numerous drop and overside tests proved 
the Mark IV design acceptable. On one occasion 
a submarine at periscope depth was tracked for 
2 hours by a PBM plane using two directional 
buoys. The buoy observations were later checked 
against the course as shown by the submarine 
log and against bearings based on visual ob- 
servations from the plane. These are plotted in 


C' 


THE AN/CRT-4(XN-1) PRODUCTION BUOY 


91 



Figure 21. Comparison of DRSB bearing indications with other determinations. 


Figure 21. It will be noted that the bearing 
error rarely exceeded 10 degrees. 

THE AN/CRT.4(XN.l) PRODUCTION 
BUOY 

The production buoy is essentially the Mark 
IV. It is 521/^ inches long, 6 % inches in diame- 
ter, and weighs 30 pounds, including batteries. 
In its closed condition, as shown in the cutaway 
section of Figure 22, the antenna is folded 
down along the side of the buoy housing and 
the hydrophone and motor assemblies are en- 
closed within the bottom section of the housing. 
During the launching operation these compo- 
nents automatically extend, the antenna erect- 
ing to approximately 40 inches above the top 
deck of the buoy during the drop and the hydro- 
phone and motor assemblies descending to 
about 8 feet below the surface of the water 
after impact. The buoy, with components ex- 
tended, is shown in Figure 23. 


^ Buoy Housing 

The tubular housing is of bakelite-impreg- 
nated paper, surmounted by a cap of phenol 
fabric, and consists of two separate tubes, 
rigidly attached end to end to provide two 
distinct compartments. The upper one of these, 
which contains the radio chassis and batteries, 
is watertight and constitutes a buoyancy cham- 
ber. The lower compartment houses the folded 
hydrophone and motor mechanism. The top 
cap provides storage space for the packed para- 
chute and its lines and a seamarker dye packet. 
In addition it serves, up to the moment of 
launching, as a cover for the equipment 
mounted on the top deck of the transmitter 
section and provides a means for clamping the 
antenna in its folded position. 

The upper section is closed at the bottom 
by a brass waterseal bulkhead in which is 
mounted the receptacle of a watertight separa- 
ble connector for connection to the hydrophone. 
The aluminum deck at the top supports the 




92 


AIRCRAFT LISTENING EQUIPMENT RADIO SONG BUOYS 



Figure 22. AN/CRT-4 (XN-1) buoy, cutaway view. 

transmitter chassis and the antenna and, with 
the help of a rubber gasket, completes the 



Figure 23. AN/CRT-4 (XN-1) buoy with compo- 
nents extended as after launching. 

watersealing of the buoyancy chamber. To 
provide electric shielding and thus aid in 
stabilizing the frequency of the transmitter, 
the outside of this upper section is sprayed 
with atomic zinc which is grounded to the 
transmitter chassis. 


CO 


THE AN (:KT-1(\N-I) PRODUCTION BUOY 


93 


The lower section of the housing has a metal 
bulkhead mounted at the top to support the 
telescoping tube to which the hydrophone as- 
sembly is attached. The bulkhead also provides 



Figure 24. Top assembly removed from housing. 


accommodation for the coiled hydrophone cable. 
A nonmagnetic metal band connects the two 
sections of the housing, and another such band 
reinforces the bottom edge of the lower section. 

Each individual buoy is color-coded to indi- 
cate the operating frequency to which its trans- 
mitter was tuned at the factory. The painted 
stripes just below the top of the housing pro- 
vide this information. 

As in the ERSB, for purposes of security a 
soluble plug of Carbowax, which seals a hole 
in the top deck, dissolves in a few hours and 
the buoy sinks after its useful life is ended. 


Transmitter Section 

Figure 24 shows the top assembly removed 
from its housing and Figure 25, the top deck 
in some detail. 

Antenna 

I 

Components of the antenna are shown in 
Figure 26. The antenna consists of a spring- 
tempered steel rod 30 inches long attached to a 
flat stainless-steel spring 9 inches long by IVb 
inches wide. The spring section is made of 
three leaves each bowed slightly in the IVa 
inch dimension. A rubber boot, enclosing the 
spring section, insures proper insulation and a 



Figure 25. Top deck details without dye pack. 


cushion qf sponge rubber maintains sufficient 
spacing between antenna and water to mini- 
mize capacity variations. 

The toggle-type battery switch is spring- 
actuated, with its on position the normal one. 
It is held in the off position when the antenna 
is folded down against the buoy. 





94 


AIRCRAFT LISTENING EQUIPMENT RADIO SONO BUOYS 



Figure 26. Antenna components. 


Transmitter Chassis 

Except for slight modifications, the transmit- 
ter chassis is similar to the type used in the 
nondirectional buoy. Components are mounted 
on the two sides of a vertical metal plate with 
the compass at the bottom. This unit is sus- 
pended by rubber shock mountings within a 
nonmagnetic framework and, together with the 
battery pack, is clamped to the top deck in such 
a manner as to allow for precise orientation of 
the compass with respect to the hydrophone at 
the time of assembly. A transparent acetate 
tube surrounds the transmitter for protection 
during adjustment and installation of batteries 
in the field. 

Transmitter Circuit 

The circuit of the frequency-modulated 
transmitter is given in Figure 27. Briefly it 
consists of a two-stage a-f amplifier followed 
by a reactance-tube modulator, oscillator, and 
amplifier. Audio-signal voltages applied to the 
grid of the reactance tube vary its transcon- 
ductance and hence the reflected reactance 
across the oscillator tank circuit. This in turn 
modulates the oscillator frequency which can 
be adjusted over the range, 15.5 - 18.0 me. 
The plate circuit of the oscillator tube provides 
frequency doubling, as well as the amplifier 
tube. The output center-frequency range is thus 
62 - 72 me. This frequency double-doubling 
arrangement eliminates the necessity for com- 
plex neutralizing circuits and also, by quadru- 
pling the frequency deviation due to modula- 
tion, provides a 12-db improvement in signal- 
to-noise ratio. 



THE AN CRT-t(XN-l) PRODUCTION BUOY 


95 



SLAVE - 
ELEMENT 


’MASTER ■ 
ELEMENT 




MAGNETS 


Figure 28. Compass-capacitor and components. 


Orientation System 

The compass-capacitor incorporated in the 
transmitter is of the slave-master type. Details 
of its construction are shown in Figure 28. Two 
magnets which act as the driving element are 
suspended by means of a single-pivot jewel 
bearing and rotate freely even though the case 
is tilted up to an angle of 25 degrees. An 
eccentric metal plate and magnet mounted on 
a shaft pivoted in jewel bearings at both ends 
comprise the slave element. The eccentricity of 
the plate is such that as it rotates the capacity 
between it and the stator fork which straddles 
it varies continuously over the range 2.5 to 2.8 
/xf. This capacity is connected as part of the 
oscillator tank circuit. The 0.3-/xf variation 
provides a frequency swing of ±75 kc in syn- 
chronism with buoy rotation. 

Bottom Section 
Hydrophone Support 

The telescoping torque tube for supporting 
the hydrophone and motor assembly is shown 
in Figure 29 and in sectional drawing in Fig- 
ure 30. It consists of seven sections of brass 
tubing of graduated diameters and measures 
about 12 inches long when collapsed and 68.5 
inches extended. Thin spring sleeves in the 
form of open bands ride between the tubes and 



Figure 29. Telescoping support tube. 





96 


AIRCRAFT LISTENING EQUIPMENT RADIO SONO BUOYS 



SPRING SLEEVE 


TIPS SWAGED TO 
SMALLER DIAMETER 





BUTTS EXPANDED _____ j 


Figure 30. Sectional drawing of telescoping support tube. 


provide a wedge between adjacent tubes when 
the system is fully extended. These lock the 
sections securely into position, increasing stiff- 
ness and minimizing undesirable noise. 



Figure 31. Bottom assembly with hydrophone 
partly unfolded. 


Hydrophone 

Attached to the lower end of the torque tube 
is the hydrophone and motor assembly. Figure 


31 shows the hinged halves of the hydrophone 
falling into operating position, to be held in 
horizontal line by a spring hook. The motor 
blades, aided by spring hinges, have already 
assumed their operation positions. 

Each section of the hydrophone consists of a 
12-inch length of annealed seamless nickel 
tubing, 1% inches in diameter and 0.025-inch 
in wall thickness, on which 83 turns of Vinylite- 



RUBBER 

SHOCK MOUNTING 


Figure 32. Cross section of hydrophone. 


THE AN/CRT-t(\IN-l) PRODUCTION BUOY 


97 


insulated wire are wound toroidally. The tube 
is then lined with a thin layer of cellular 
rubber, backed with an aluminum baffle plate, 
and magnetized by discharging a large ca- 
pacitor through the toroidal winding. A cross- 
sectional drawing of this structure is shown in 
Figure 32. 

The rubber lining and baffle plate provide 
improved directional properties of the hydro- 
phone in both the horizontal and vertical 
planes. Its pattern described in Chapter 6, 
Volume 11, Division 6, in the horizontal plane is 
shown in Figure 33. To gain the full directional 
effectiveness of the hydrophone, it is essential 
that the two sections be as nearly identical as 
possible. The two nickel tubes are therefore 



adjacent sections from the original tubing 
stock, are heat-treated in the same manner, and 
are assembled in their original relative posi- 
tions in the final hydrophone. 

Motor Assembly 

A close-up of the motor assembly. Figure 34, 
shows the reel and line, line guides, paddle. 


and motor weight. The bottom plate of the buoy 
when detached serves as the weight, causing 
the nylon line to unwind and rotate the reel 
and hydrophone at about 4 rpm. The relatively 
large blade surface of the paddles provides the 
necessary back pressure. 

Because of the close rnechanical and acoustic 
coupling between the motor and the amplifier 
and because of the large voltage gain of the 
amplifier (about 100 db), it is extremely im- 
portant that the total noise from the drive 
mechanism be as low as possible. So critical is 
this that ball bearings could not be used in the 
reel, and a special sleeve bearing is provided. 
Other factors which contribute toward quiet 
operation include composition rollers in the line 
guide, a positive latch for locking the two sec- 
tions of the hydrophone in line, strong springs 
in the paddle hinges, shock-mounting of the 
hydrophone baffles, and the special joint con- 
struction of the torque tube described above. 



Figure 34. Motor assembly and hydrophone lock. 


98 


AIRCRAFT LISTENING EQUIPMENT RADIO SONO BUOYS 


Besides serving as the motor weight during 
operation, the base plate is the protecting 
closure for the bottom of the buoy. It consists 
of two parts, a perforated outer safety plate 
and the inner release disk held together by 
spring hooks. Release springs attached to the 
buoy housing hold the inner disk in position, 
as shown in Figure 35. On impact with the 



Figure 35. Bottom plate at instant of automatic 
release. 


water this disk is forced upwards, the release 
springs unhook, and the hydrophone and motor 
assemblies fall into operating position. 

A special bottom plate which can be triggered 
by hand is provided for launching from surface 
craft. 

^ Cap Section 

The molded phenol fabric top cap of the 
buoy houses the antenna components and dye 
pack and serves to hold the antenna in its 
folded position. At the bottom edge of the nose 
of the cap is a hinged flap which holds the an- 
tenna against the side of the buoy and thereby 
keeps the battery switch in its off position. Re- 
leasing this flap permits preflight testing of the 
buoy. 


On launching, one end of a static line is 
attached to the aircraft, the other to the static 
tape which passes through the top of the cap 
to a key. The key in turn is connected to the 
top of the parachute. As the cap is pulled off 
and the parachute emerges, the key turns and 
static line slides out, thereby freeing the para- 
chute and casting off the cap. The antenna also 
erects during this operation, thus turning on 
the transmitter. 

Parachute 

The parachute is of nylon, 36 inches in diam- 
eter, with twelve 6-foot rayon shrouds ter- 
minating in a 15-foot load line. The other end 
of the load line is attached to the top deck of 
the buoy through a hydraulic-delay release 
which serves to cast off the parachute on im- 
pact with the water. 



RELEASED 


Figure 36. Stages in operation of parachute release. 

Three stages in the operation of the para- 
chute delay mechanism are shown in Figure 36. 
As the parachute blossoms the plunger is 
pulled downwards, but the hydraulic delay is 
such that about 3 seconds are required for it to 
reach its extreme position. During this time the 
side hooks are held locked under the cable but- 
ton by the confining action of the barrel wall. 


THE AN/ARR-16 RECEIVER 


99 


This delay allows for the first few moments of 
erratic motion following parachute opening 
after which the pull becomes steady. When the 
buoy reaches the water, the pull is removed, the 
side hooks spring out, and the parachute and 
release mechanism are discarded. 

Dye Pack 

The dye pack, consisting of standard life- 
jacket fiuorescein powder enclosed in a cotton 
bag, is sealed in a Vinylite-impregnated bag 
which is tied to the top of the buoy. One side 
of the bag is unsealed during launching, and 
thus coloring of the water starts as soon as the 
buoy strikes the water and assists in locating 
the buoy visually from the air. Figure 17 shows 
the position of the bag in the water and the dis- 
coloration produced. Active life of the dye pack 
in water is from 1 hour to 6 hours, depending 
on water conditions. 

’’ THE AN ARR-16 RECEIVER 

The AN/ARR-16 receiver used with the 
DRSB is a modification of the AN/ARR-3A 
receiver employed with the nondirectional buoy. 
This modification involves principally the ad- 
dition of a vacuum-tube voltmeter circuit as 
the direction-indicating component. 

Figure 37 shows the receiver and its acces- 
sories. These include the 24-volt power supply. 


made for plugging the plane’s interphone sys- 
tem into the receiver to enable the operator to 
receive information and commands while listen- 
ing for buoy signals. 

Receiver Circuit 

The receiver employs *14 tubes in the circuit 
of Figure 38, and tunes over the frequency 
range from 62.3 to 72.3 me. Operation of the 
bearing-indicating component will be described 
here. Other technical and operational details 
are available elsewhere in this chapter and in 
references 2 and 3. 

In the operation of the receiver, the a-f out- 
put voltage of the discriminator tube is applied 
to the a-f amplifier stages, while any slower 
variation of voltage due to carrier center-fre- 
quency shift is segregated and applied to the 
input of the AFC system. As the carrier is 
caused to shift continuously by the rotation of 
the buoy, the AFC system receives a constantly 
varying voltage which is measured by the 
vacuum-tube voltmeter circuit. The scale of the 
indicating meter is calibrated in points of the 
compass instead of in volts and thus provides 
a direct indication of the orientation of the 
buoy to which it is tuned. The face of this 
meter is shown in Figure 39. 

For accurate bearing readings it is necessary 
to tune the receiver to the precise center fre- 
quency of the carrier and to regulate the volt- 



Figure 37. The AN/ARR-16 directional buoy receiver and accessories. 


and two remote bearing-indicating meters and age applied to the vacuum-tube voltmeter cir- 
phones, one for use at any point in the plane, cuit (by the scale expander potentiometer) to 
the other for the operator. Provision is also such a value that the indicating meter just 




100 


AIRCRAFT LISTENING EQUIPMENT RADIO SO NO BUOYS 




Figure 38. Schematic circuit diagram of AN/ARR-16 receiver 


PERFORMANCE TESTS 


101 


covers the entire scale. Proper orientation of 
the compass with respect to the hydrophone is 
of course necessary. This is checked at the time 
of assembly with due allowance for the drag 
which the rotating liquid exerts on the elements 
of the compass as the unit revolves. 



Figure 39. Meter face of bearing-indicator. 


9 OPERATION OF THE EQUIPMENT 

The following is a brief description of 
launching procedure and the sequence of auto- 
matic operations that then occur. 

The buoy may be manually launched through 
a hatch or mechanically launched from the 
bomb bay. In either case, the static line (at- 
tached to the plane) is connected to the static 
tape in the cap of the buoy. As the falling buoy 
reaches the limit of the static line, the cap is 
pulled off, the antenna erects, the battery 
switch is released to turn on the transmitter, 
and the dye pack and parachute are pulled from 
the cap. This releases the static line and the 
now useless cap is discarded. The parachute 
then blossoms, the parachute release is cocked, 
and the unit is eased downwards toward the 
water. On striking the water the bottom plate 
is automatically unlatched and the parachute 
is released. The bottom assembly then drops 
out of its housing, extending and locking the 


torque tube. The hydrophone and motor paddles 
assume operating positions and the bottom 
plate, serving as motor weight, starts the buoy 
rotating. 

The effective operating life of the buoy 
depends on water depth, on the amount of line 
wound on the motor reel, and on the battery 
supply. The motor reel \is wound with about 
500 feet of line which is expended at about 100 
feet per hour. In deep water it therefore rotates 
for about 5 hours, which is also the life of the 
battery pack under best conditions. If the bat- 
teries are aged, or if ambient temperature is 
low, their life may be as low as 2 hours. 

Before a buoy is launched, its carrier fre- 
quency is ascertained from its identifying color 
band, and the receiver may be set to this fre- 
quency at once. When the buoy reaches the 
water it immediately starts to relay underwater 
sounds, and if any submarine is within sound 
range, its bearing from the buoy can be deter- 
mined. To locate the submarine more precisely, 
triangulation can be employed by dropping a 
second buoy transmitting at a different fre- 
quency. 

’ll PERFORMANCE TESTS 

On completion of the first hundred produc- 
tion buoys, the Navy arranged to make com- 
prehensive tests to determine their operating 
characteristics. Tests were made over an ex- 
tended period of time and under various con- 
ditions of the sea. Besides the necessary sur- 
face craft, eight aircraft and five submarines 
(two of them Italian) participated in the 
program. 

Submarine Targets 

A summary of the results on location of sub- 
marines is given in the graphs of Figures 40 
and 41. Although all submarines employed as 
targets were in excellent condition and defi- 
nitely quiet in operation, it was necessary to 
discard a large number of observations because 
of noise which developed in some of the sub- 
marines during the tests. Maximum buoy 
ranges resulting from shaft squeals or other 
unusual sounds are therefore not included in • 
the summary. 






102 


AIRCRAFT LISTENING EQUIPMENT RADIO SONO BUOYS 




Figure 41. Bearing accuracy of DRSB. 


The average maximum range at which vari- 
ous submarines could be detected with the 
DRSB is given in Figure 40 for various states 
of the sea and for different submarine speeds. 
To ascertain this range from the graph, follow 
a horizontal line from the number on the left 
corresponding to the speed of the submarine 
concerned over to the point where it intersects 
•the line representing the state of the sea and 
read the position of this point on the range 



Figure 42. DRSB maximum detection ranges for 
surface craft. 


scale. For instance, the Lion Fish, at 10 knots, 
could be picked up at 3,000 yards in a quiet sea 
and at about 700 yards in a state 5 sea. 

The graph of Figure 41 indicates that the 
average observer attains an accuracy of about 
dzlO degrees in bearing determination. 

At the frequency used by the DRSB, radio 
range approximates line-of-sight and therefore 
depends on the elevation of the aircraft. Actual 
ranges determined were: 6 miles at 200 feet 



PERFORMANCE TESTS 


103 


elevation; 11 miles at 500 feet; 16 miles at 
1,000 feet; and 26 miles at 3,600 feet. 

Surface Craft Targets 

Figure 42 presents a summary of data on 
the effective sonic range of the DRSB against 
various antisubmarine warfare [ASW] craft, 
both as a function of target speed and state of 
the sea. This figure is read in the same manner 
as Figure 40. 

DRSB FROM Surface Craft 

Use of the nondirectional ERSB by surface 
craft requires that the ship remain quiet or 
move very slowly. Because of the directional 


characteristics of DRSB, it was found that a 
surface ship could move in toward a buoy or 
target and still not produce too much interfer- 
ence if it laid its course to avoid a buoy-ship 
bearing within about 30 degrees of the buoy 
target bearing. This suggests the possibility of 
closer cooperation between the air and surface 
components of a composite ASW task group 
than is possible with the nondirectional ERSB. 

The radio range of the buoy to a receiver 
aboard a surface ship was found to be limited 
to about 5 miles because of the relatively small 
elevation of the buoy antenna. 

As a result of the above tests, the adoption 
of the DRSB was recommended and an original 
procurement of 1,000 units was increased to 
7,000. 




Chapter 10 


SUBMARINE LISTENING EQUIPMENT--JP AND JT SYSTEMS 


10.1 INTRODUCTION 

E arly in the submarine listening program, 
the use of existing sonic equipment had 
been abandoned as ineffective because of the 
limited frequency response and poor direction- 
ality of available hydrophones, the high sub- 
marine self-noise levels in the sonic frequency 
range, and the lack of adequate control of 
amplifier frequency response characteristics. 
At the time of our entry in World War II, U. S. 
submarines 'were capable of listening in the 
supersonic range only. The equipment was cap- 
able of providing accurate bearings and detect- 
ing, at a considerable range, noise produced by 
propellers turning at above cavitation speeds. 
However, the characteristic noise peaks which 
make it possible for an experienced sound op- 
erator to identify different types of vessels at 
long range are largely in the sonic frequency 
band. In addition the noise produced by station- 
ary or slowly moving vessels is generally of ma- 
chinery origin with few, if any, components in 
the supersonic region. Further, as sonic fre- 
quencies are subject to less attenuation in sea 
water, they can be detected at greater maxi- 
mum ranges. A sonic system was therefore 
desirable for early detection and identification 
of the target. In addition, the supersonic trans- 
ducers on most fleet-type submarines are 
mounted near the keel, where it is impossible 
to lower the head for listening if the boat is 
lying on the bottom. Under this condition, a 
submarine equipped with only supersonic gear 
is without acoustic means for obtaining infor- 
mation about surface vessels in the vicinity. 
For this reason, both JP and JT gear provide 
topside mounting of the hydrophone. 

Work on topside submarine equipment pro- 
gressed with these ends in mind, to identify 
types of ships and to detect sounds produced 
by stationary or slowly moving surface craft, 
along with the additional aim of indicating 
any detectable noise produced by the submarine 


itself. Research made possible the develop- 
ment of toroidal and straight magnetostriction 
hydrophones of rugged and simple construc- 
tion possessing good wide-band frequency re- 
sponse characteristics together with good di- 
rectionality.^’2 Preliminary tests showed that 
listening with these hydrophones was possible 
at speeds of from 3 knots to 6 knots. They also 
indicated that, if the hydrophone could be 
made directional, an effective sonic listening 
system for submarines was feasible. 

Development work was consequently under- 
taken and proceeded in parallel with the de- 
velopment of the directive sonic listening equip- 
ment for small patrol craft discussed in Chap- 
ters 4 through 7. This work led first to the 
design and construction of the JP-1, JP-2, and 
JP-3 sound receiving equipment for sub- 
marines and later to the development of the 
JT sonar equipment. 

The factors controlling the listening range 
of such equipment are highly variable. Under 
very favorable conditions, surface ship pro- 
peller sounds have been heard with the JP 
hydrophone at ranges in excess of 20,000 yards 
and the auxiliary machinery of destroyers de- 
tected up to approximately 1,000 yards. How- 
ever, it is believed that the effectiveness of the 
JP equipment is reduced by the structural 
transmission of vibration from the submarine 
to the hydrophone through the training shaft, 
by waterborne noise from the propulsion ma- 
chinery, auxiliaries, and superstructure of the 
submarine, and by the considerable effort 
necessary to train the hydrophone by hand for 
extended periods of time. 

As tactical information concerning the sonar 
needs of the submarine forces became available 
through reports of patrol activities, plans were 
considered for improving either the existing 
JP or the WCA gear in use. Although both 
modification plans presented were judged to be 
equally desirable from a tactical standpoint, it 
was believed that changes made primarily to 


104 


GENERAL DESIGN CONSIDERATIONS 


105 


the JP system would result in faster produc- 
tion and less installation-time requirement. 
Improvements include the use of a split delobed 
hydrophone and right-left indicator [RLI] 
system to permit more accurate determination 
of bearings and incorporation of a supersonic 
converter to allow sonic or supersonic tracking 
of targets. Continuous rotation and power- 
training of the hydrophone, a two-way talk- 
back system, and various other new features 
combined to reduce operator fatigue and to 
attain closer coordination between the sound 
operator and the attack team. 



JP-lf JP~2^ aad JP-3 Equipment 

The JP-1, JP-2, and JP-3 equipment is de- 
signed to he used on submarines to detect sur- 
face ship sounds and give their relative hearing 
from the submarine and to monitor submarine 
self-noise. It comprises a 3-foot magnetostric- 
tion wood-core line hydrophone and baffle, a 
sonic listening amplifier, and a hand-training 
mechanism which can he used ivith the sub- 
Tnarine underway at 3 to Ji. knots submerged. 


To permit operation while the submarine is on 
the ocean bottom, the hydrophone is mounted 
topside on a shaft which extends through the 
forivard torpedo room pressure hull. With its 
associated baffle, the hydrophone is directional 
in the horizontal plane and its response rises 
ivith frequency at the rate of approximately 
6 db per octave from a value of about -115 
db vs 1 volt per dyne per sq cm at 1,000 c. The 
amplifier is equipped with various high-pass 
filters to aid in discriminating against ambient 
and self-noise and to increase the directivity for 
accurate bearings. Possible bearing accuracy 
on certain types of noise sources is ivithin ap- 
proximately 1 degree. Surface ship propeller 
noise can be heard at ranges out to 20,000 
yards under good weather conditions and de- 
stroyer auxiliaries out to 1,000 yards. This 
equipment was developed by CVDWR-NLL. 

general design considerations 

For a submarine’s sonic listening system to 
be most useful, a number of specific require- 
ments must be met. 

1. Its listening range for propeller sounds 
should be several thousand yards and auxiliary 
machinery noise should be detectable at a range 
of several hundred yards under good conditions. 

2. It should have sufficient directivity to 
discriminate against the noise from the sub- 
marine’s own propellers at low speeds and to 
locate target bearings with reasonable ac- 
curacy. 

3. Its hydrophone and amplifier should have 
good response characteristics in the lower fre- 
quency range (down to about 100 c) to detect 
the components in this region from surface 
craft auxiliary machinery. 

4. The amplifier should be provided with 
high-pass filters to aid in the accurate deter- 
mination of bearings and to discriminate 
against the noises of the submarine’s own ma- 
chinery when necessary. 


» JP-2 and JP-3 sound receiving equipment are sub- 
sequent models of the JP-1 and embody only relatively 
minor changes. The term JP is used informally in the 
text to refer to any or all of the models. This equip- 
ment should not be confused with the JP overside and 
through-the-hull equipment for small patrol craft dis- 
cussed in Chapter 7. 




106 


SUBMARINE LISTENING EQUIPMENT JP AND JT SYSTEMS 


5. Its dimensions should be minimal because 
of the space limitations on all submarines. 

6. In so far as feasible its components should 
operate directly from the ship's d-c power 
supply to facilitate use during evasive maneu- 
vers. 

10.3 EARLY MODELS 

In the first model of the topside-mounted 
sonic listening equipment, the toroidal mag- 
netostriction hydrophone was fitted with a 
ring-shaped iron baffle backed with a cork and 
rubber compound to provide front-to-back dis- 
crimination. The amplifier, adapted from small 
patrol craft listening equipment, was modified 
to provide for attenuation of the higher fre- 
quencies and to permit a diode rectifier to be 
used in the listening channel when desired. 
The amplifier loss at higher frequencies gave 
the system an overall frequency response that 
was essentially flat over the sonic region. The 
diode rectifier arrangement, used in conjunc- 
tion with a 5,500-c high-pass filter available in 
the amplifier, was found to aid the operator 
in taking propeller turn counts and also to give 
improved bearing accuracy under some condi- 
tions. Power for the amplifier was supplied 
by a special set of A and B batteries furnished 
with the equipment. 

Six units of this model were installed on 
R-class submarines, utilizing the training shaft 
of the topside-mounted JK supersonic gear 
with which these boats are equipped. Exten- 
sive tests, designed to evaluate the character- 
istics of the sonic equipment and to compare 
its performance with that of the JK supersonic 
gear, were conducted in deep water off Key 
West and in the comparatively shallow water 
of Long Island Sound with the following 
results. 

1. No essential differences were detected in 
the performance of the sonic gear in deep and 
shallow water areas. 

2. Sounds from surface ship auxiliary equip- 
ment were observed and frequently identified 
by sonic listening at distances up to several 
hundred yards under average listening condi- 
tions. 

3. Propeller sounds were generally detected 


at greater distances with the sonic than with 
the supersonic equipment. 

4. The capabilities of the sonic and super- 
sonic systems to determine the bearing of a 
target were essentially equal. 

5. The ability to identify signals and to 
take propeller turn counts on the sonic equip- 
ment was equal or superior to that on the 
supersonic gear. 

6. At submarine speeds in excess of 3 knots, 
interference from the submarine’s own screws 
was found to be slightly greater on the sonic 
than on the supersonic system. 

7. The sonic equipment was found to have 
slightly less front-to-back discrimination than 
the JK equipment but had no ambiguity of 
direction. 

A second model was constructed for tactical 
use on P-class submarines (also having topside- 
mounted JK gear) and for test installations on 
two new-construction boats. This model utilized 
substantially the same hydrophone as the 
earlier units but was provided with an im- 
proved amplifier having considerably more 
gain at frequencies below 500 c and powered 
from the ship’s batteries using a line filter to 
suppress transients. 

The new-construction submarines, having no 
topside-mounted JK gear, required the provi- 
sion of separate training mechanisms for the 
sonic gear. Some difficulty, due to binding of 
the shaft at deep submergence and to noise 
caused by tight packing glands, was encoun- 
tered with these mechanisms. 

10.4 PRODUCTION MODEL 

JP-I EQUIPMENT 

Patrol reports from submarines equipped 
with topside-mounted sonic listening equip- 
ment confirmed the tactical effectiveness of this 
type of gear. Continued development work re- 
sulted in the design and construction of a new 
model, designated the JP-1 sound receiving 
equipment, intended primarily for installation 
on new-construction submarines. The JP-1 
equipment utilizes a straight magnetostriction 
hydrophone in place of the former toroidal 
unit and incorporates a number of other 
improvements over the earlier models. 


PERFORMANCE 


107 


Hydrophone and Baffle 

The straight magnetostriction hydrophone 
used with the JP-1 equipment is of wood-core 
construction, 3 feet long, and is mounted 30 
inches above the submarine's deck. The unit 
is equipped with an acoustic baffle consisting 
of a hollow, free-flooding bronze casting 
covered on the back by a blanket of air-fllled, 
noncommunicating-cell neoprene. 

The hydrophone has a front-side open-circuit 
sensitivity of approximately -115 db vs 1 volt 
per dyne per sq cm at 1,000 c. Its response 
rises with frequency at the average rate of 
about 6 db per octave in the region between 
200 and 10,000 c. The baffle provides a front- 
to-back discrimination which varies from about 
5 to 15 db in the range of 500 to 10,000 c. 

Mounting and Training Mechanism 

The hydrophone mounting and training mech- 
anism, designed and manufactured by the 
Navy, varies somewhat in detail, according to 
the class of submarine for which it is intended. 
It consists essentially of a handwheel geared 
to a watertight hollow vertical shaft on which 
is mounted a simple azimuth indicator. The 
hydrophone cable enters the pressure hull 
through the shaft. 

Amplifier ' 

The amplifler, installed near the after bulk- 
head of the forward torpedo room, is designed 
to operate from a d-c power supply of 120 volts. 
It is of the resistance-coupled type, employing 
negative feedback between the first ‘two and 
the last two stages with a push-pull output 
stage to provide ample power for listening at 
the low frequencies. An output transformer 
with a nominal impedance of 300 ohms pro- 
vides for low-impedance headphone or loud- 
speaker monitoring; a high-impedance output 
is provided for crystal headphones. The voltage 
for the tube heaters, which are connected in 
series, is controlled by a voltage regulator tube 
which permits operation on line voltages vary- 
ing from 85 to 130 volts. A set of Alters is 
provided between the second and third stages 
for controlling the frequency characteristics of 


the amplifier. By means of these filters the 
low frequencies may be emphasized or sup- 
pressed as necessary to reduce unwanted noise 
and improve the operator’s ability to hear the 
desired signal. 

Visual detection based on the higher fre- 
quency components of received signals is pro- 
vided by an electron ray (magic-eye) tube in 
the amplifier. By switching in a diode rectifier, 
the operator may introduce harmonic distor- 
tion into the listening circuit. The distortion 
produced in this way frequently improves the 
distinctness of propeller turn counts. 

Associated with the amplifier is a circuit for 
magnetizing the hydrophone. This circuit con- 
sists of a bank of condensers, charged by the 
main power supply voltage and discharged 
through the hydrophone coil by means of a key 
switch. 

A filter consisting of r-f and a-f sections in 
tandem is provided for the power supply. Both 
circulating and longitudinal noise currents 
are suppressed in the battery supply line. A 
typical JP-1 forward torpedo room installation 
is shown in Figure 2. 



Figure 2. JP-1 installation in forward torpedo 
room. 


PERFORMANCE 

Tests and patrol reports indicate that the 
inherent noise and sensitivity of the JP-1 sys- 
tem are such that the detection ranges obtained 
are limited principally by self and ambient 
noise or thermal gradient conditions of the 
water. Where thermal gradients are the con- 






108 


SUBMARINE LISTENING EQUIPMENT JP AND JT SYSTEMS 


trolling factor, the ranges obtained with sonic 
and supersonic equipment on propeller sounds 
are about equal. In the absence of controlling 
gradients, the JP-1 equipment usually gives 
somewhat greater ranges than the supersonic 
gear. 

Noise 

At speeds below about 5 knots, listening is 
not seriously affected by noise from turbulence 
around the JP-1 hydrophone or by noise from 
the submarine’s propellers except when the 
boat is accelerating rapidly. Of greater signifi- 
cance is noise from power operation of the bow 
planes, stern planes, and steering and from 
certain rotating equipment within the subma- 
rine. Some of the noise from internal sources 
is structurally transmitted to the hydrophone 
through the training shaft. 

Bearing 

The directionality of the system enables an 
experienced operator to determine bearings 
within approximately zbl degree on a source, 
such as cavitation noise, having considerable 
energy in the higher frequency region. 

Hydrophone 

The hydrophone and baffie are sufficiently 
rugged to stand up under ordinary service con- 
ditions. They have been tested at hydrostatic 
pressures of over 450 psi without observable 
effect and have been subjected to underwater 
explosions of depth charges at close range. In 
one instance such an explosion somewhat flat- 
tened the hydrophone but its performance was 
found to be unimpaired after remagnetization. 

Noise Monitoring 

The originally contemplated use of the JP-1 
equipment as a monitor for detecting self-noise, 
although very helpful in many instances, did 
not prove entirely successful because the single 
hydrophone was insufficiently sensitive to 
sounds originating in certain portions of the 
boat, particularly aft of the conning tower. 
For this reason a separate development pro- 


gram was initiated which led to the design and 
construction of supplemental equipment known 
as the noise level monitor [NLM] This equip- ^ 
ment provides a metering and switching 
adapter unit for the JP amplifier and utilizes 
four small hydrophones mounted at intervals 
along the pressure hull. 



Figure 3. JT sonar equipment. 


JT Sonar Equipment 

The JT sonar equipment developed by 
CUDWR-NLL is designed to detect and deter- 
mine the hearing of surface ships from sub- 
marines. Its 5-foot hydrophone is an electri- 
cally split, permanent-magnet magnetostriction 
unit, with lobe reduction. A sonic listening 
amplifier normally uses the sum of the signals 
from the hydrophone halves. An RLI unit uses 
the sum and difference signals to present a 
meter indication of hydrophone deflections off 
target. Power training and bearing repeater 
mechanisms provide hydrophone rotation at 
speeds of up to Jf.5 rpm and transmission of 
hydrophone bearings to the conning tower. A 
talkback system provides two-way voice com- 
munication between the forivard torpedo room 
and the conning tower and transmits sonar 
signals from the listening amplifier to the con- 
ning tower. 

106 DEVELOPMENT 

The JT development was a process of evolu- 
tion in which each functional component was 

6 Described in Division 6, Volume 18. 





DEVELOPMENT 


109 


carried through successive stages of research, 
design, construction, and tests of models, with 
the preparation of specifications proceeding as 
rapidly as possible. At intervals, combinations 
of components were assembled and tested to 
keep the complete system in balance. Although 
in practice there were no sharp lines of de- 
marcation between apparatus and system 
development, the development of each major 
component can be discussed separately. 

Hydrophone 

The following hydrophone characteristics 
were considered to be of primary importance: 
(1) adequate sensitivity and smooth response 
over a wide frequency range, (2) maximum 
directivity consistent with a practically useful 
size and the frequency band to be used, (3) 
adaptability to use with an RLI system, (4) 
reduced side lobes and, (5) elimination of the 
necessity for periodic remagnetization. 

Data and experience indicate conclusively 
that, for long-range detection of surface vessels, 
the u^ of sonic frequencies is preferable to the 
use of supersonic frequencies. As it was desired 
that the detection ranges obtainable with the 
JT system be at least equal to those of the JP 
equipment, it was decided to continue use of the 
band from approximately 0.5 to 10 kc for 
search listening. The range 5 to 9 kc was 
selected, on the basis of analysis and prelimi- 
nary tests, as suitable for operation of the RLI 
for accurate bearing determination. In these 
ranges, the characteristic of the straight mag- 
netostriction hydrophone is smooth, rising with 
octave, which compensates for the normal drop 
frequency at the rate of about 5 to 6 db per 
of 5 to 6 db per octave in the characteristic 
of ship’s screw noise and normal background 
water noise. 

Work was consequently directed toward im- 
provement of the JP magnetostriction hydro- 
phone and led to the development of the NL-124 
unit*" which was selected as suitable for use 
with the JT system. This hydrophone, a per- 
manent-magnet magnetostriction unit 5 feet 
long, is electrically split into two halves for 
use with an RLI unit and is made up of 10 
sections wound to yield a side-lobe reduction 

® Described in Division 6, Volume 13. 


(difference between responses of the main lobe 
and the greatest side lobe) of approximately 
25 db for a single frequency tone. The sensi- 
tivity of the NL-124 unit averages about 15 
db higher than that of the JP hydrophone, thus 
making the signal-to-noise requirements of the 
first amplifier stage less stringent and allowing 
the use of lower overall gain in the system. The 
construction details of the NL-124 hydrophone 
are indicated in Figure 4 and the frequency 
response characteristics of a typical unit with 
and without a baffle are shown in Figure 5. 

Later tests were made on another hydro- 
phone similar to the NL-124 but wound with 
less taper to provide an intermediate value of 
lobe reduction. In this unit the greatest minor 
lobes were approximately 19 db less sensitive 
than the main lobe. Measurements using the 
noise band 5 to 9 kc showed the width of the 
main lobe at the — 3-db point to be only 11 per 
cent greater than in the case of a unit without 
lobe reduction, whereas in the fully delobed 
NL-124 hydrophone the width of the main lobe 
is increased 20 per cent. On the basis of these 
tests it was decided that a lobe reduction 
greater than 18 to 20 db could be attained only 
by disproportionate increase in the width of the 
main lobe. However, since steps had already 
been taken toward production of the NL-124 
hydrophone, this unit was retained for use with 
the JT system. 

Baffle 

In developing a baffle for use with the 
NL-124 hydrophone it was considered desirable 
(1) to increase the front-to-back discrimination 
over that provided by the JP baffle and (2) to 
improve its streamlining. 

Calculations indicated that for maximum dis- 
crimination above 500 c the cross section of 
the baffle should be at least 4 inches in height 
by 10 inches in depth, including the 2 V 2 -inch 
diameter of the hydrophone. Acoustic and 
hydrodynamic tests of models led to the con- 
clusion that a teardrop section of these pro- 
portions would most adequately meet the re- 
quirements in both respects.^ 

An experimental cast-bronze baffle, weigh- 
ing 150 pounds, was discarded in favor of 
a lighter unit fabricated from stainless-steel 


no 


SUBMARINE LISTENING EQUIPMENT JP AND JT SYSTEMS 


sheet stock. This baffle, with the same shape, of %-inch Cell-tite rubber. The directivity pat- 
dimensions, and equivalent strength character- terns (with halves connected series aiding and 
istics, weighed only 48 pounds. As with the JP series opposing) of the NL-124 hydrophone 




.025" ANNEALED NICKEL 
RES INOX CASTING 



CARDBOARD 
MAGNET (ALNICO) 
NEOPRENE 



CARDBOARD 
IMPREGNATED WITH RESINOX 


WINDING- 


RESINOX CASTING 



Figure 4. Construction details of JT hydrophone. 



Figure 5. Frequency response characteristics of 
JT hydrophone with and without baffle. 


baffle, the metal fairing is hollow and free 
flooding. Pressure release is provided by a 
blanket made of i/i e-inch neoprene over a flller 


equipped with this baffle are shown in Figure 6. 
These were measured using a 5- to 9-kc noise 
band as a source and indicate an improvement 
of 6 db in front-to-back discrimination com- 
pared to that of the JP hydrophone and baffle 
in the same frequency band. 

Sound Absorbing Coupler 

Hull vibration transmitted to the hydrophone 
through the metallic support and training sys- 
tem of JP installations limited target detection 
from many of the noisier submarines. Several 
methods were considered for isolating the 
hydrophone from vibration or shock applied to 
the shaft without loss of sufficient torsional 
stiffness to retain the bearing accuracy of the 
system. 

It was decided on the basis of preliminary 




DEVELOPMENT 


111 


experiments to use a sandwich-type coupler, 
consisting of two brass plates or disks with a 
layer of rubber between them. Calculations 
were made to determine the theoretical attenu- 
ation produced by various grades and thick- 
nesses of rubber. These indicated that approxi- 
mately 1 inch of rather soft rubber (25-30 
durometer) would be required to suppress fre- 
quencies down to 200 c. However, the best bond 
to brass was obtained with rather hard rubber 
or neoprene of at least 40 durometer. Several 


noise made at various submerged speeds indi- 
cated that at 3 knots the greatest vibration was 
in the 100-c to 200-c band, with the energy de- 
creasing at higher frequencies to a negligible 
value at 3,000 c. It was found in dockside tests 
that a 28-durometer coupler attenuated all fre- 
quencies above 170 c by 10 to 15 db and that a 
50-durometer coupler attenuated all frequencies 
above 300 c by an average of 8 db. A double 
mounting was accordingly constructed on a sub- 
marine to allow direct comparison under actual 




220* 200* I80’ 160“ 140“ 

Figure 6. Directivity patterns of JT hydrophone assembly: (Left) Series aiding and (right) series opposing. 


samples of various grades of neoprene from 30 
to 50 durometers were tested in tension and 
shear and on a shake table. In the harder 
samples the bond had a tensile strength of 340 
psi and withstood vibration of l^-inch ampli- 
tude at 1,000 c for 8 hours with no evidence of 
fatigue. Listening tests indicated that, when 
using the 500-c high-pass filter of the JP am- 
plifier, the coupler produced a noticeable im- 
provement at submarine speeds up to 3 knots. 
At higher speeds no improvement was observed, 
presumably because of turbulence or water- 
borne vibration transmitted from the hull to 
the hydrophone. 

Disk recordings of submarine background 


conditions. Listening tests and recordings were 
made at 2 knots, 4 knots, and 6 knots in rela- 
tively quiet water 100 fathoms deep. Under 
these conditions, a coupler with 25- to 30-du- 
rometer rubber provided a reduction of up to 
6 or 8 db in the background noise from auxil- 
iaries and training gear. The tensile strength 
of this brass to soft rubber bond was 120 psi, 
judged to be adequate since there were 70 
square inches of area. Nonetheless, three rub- 
ber-cushioned safety bolts provided protection 
against loss of the hydrophone through any 
possible casualty to the bond. The torsional 
stiffness of the coupler was such that the torque 
created by water forces on the JT hydrophone 




112 


SUBMARINE LISTENING EQUIPMENT JP AND JT SYSTEMS 


and baffle at the critical bearings could cause a 
maximum deflection of 0.1 degree at 3 knots 
and 0.35 degree at 6 knots. 


RLl AND Listening Amplifier 

Tracking a target with JP or WCA equip- 
ment requires the operator to sweep the hydro- 
phone beam across the target. Only intermit- 
tent bearing information, having an average 
accuracy of about ±1.5 degrees, is provided. 
An investigation of methods to improve this 
condition, for use in the triangulation listening 
ranging [TLR] system discussed in Chapter 12, 
included the analysis of several types of hear- 
ing deviation indicators [BDl] , all of which are 
based on the difference in arrival time of 
sound at the two halves of a split hydrophone. 
In the selected circuit, designated RLI, sepa- 
rate signals from the hydrophone are fed into 
a transformer network which takes the vector 
sum and difference of the signals as indicated 
in Figure 7. Before amplification, the sum and 


TARGET 







SUM 


VECTORS 



TARGET BEARS RIGHT 
POSITION 1 


INPUT TRANSFORMERS 


VECTORS 
L _ R . 


SUM 


L . .R 


DIFF 

0 

ON TARGET 
POSITION 2 


VECTORS 



TARGET BEARS LEFT 
POSITION 3 

at RLI input. 


Figure 7. Vector relationships 


difference signals are separated ± 90 electric 
degrees for the off-target conditions indicated 
as positions 1 and 3. By means of additional 
electronic equipment terminated by a zero-cen- 
ter meter, a direct visual indication of whether 
the hydrophone is on target or requires right 
or left training is presented to the operator. 


A prototype model was designed which pro- 
vided good response between 0.5 and 14 kc to 
permit extending the frequency band above the 
originally selected 5- to 9-kc band should sys- 
tem tests prove this desirable. 

A block schematic of this RLI unit (also 
representative of pilot models and production 
units) is shown in Figure 8. Signals from the 
sum and difference input transformers are am- 
plified before passing through 0.5 to 14-kc 
band-pass filters provided to attenuate 1-f noise 
and h-f echo-ranging signals. Special attenua- 
tors control the gain of the two RLI channels 
either manually or by means of an automatic 
volume control [AVC] circuit. The signals are 
further amplified and passed through 5- to 9-kc 
band-pass filters, after which phase-shifters in 
each channel produce a net advance in phase of 
90 degrees for the difference channel. The sig- 
nals in the two channels being formerly in 
quadrature, the additional 90-degree phase 
shift results in signals in the sum and differ- 
ence channels which are either in phase or 180 
degrees out of phase. 

Both channels are further amplified and the 
sum channel is phase-inverted prior to rectifica- 
tion of both signals by a phase-sensitive detec- 
tor. The rectified d-c signal is negative if the 
sound source bears to the right of the hydro- 
phone and positive if the sound source bears 
to the left. A d-c amplifier increases this signal 
to operate a zero-centered microammeter. 
When an average target signal is 1 degree off 
hydrophone bearing, the meter is deflected full 
scale. 

The listening channel is a separate amplifier 
(without AVC) which normally connects to the 
sum channel but may be connected to the differ- 
ence channel momentarily. A separate volume 
control and standard high-pass filters are pro- 
vided ; a two-stage amplifier supplies audio 
power for two high-quality dynamic headsets. 
This audio signal is also supplied to the sonar 
talkback system described in a later section. 

Supersonic Converter 

The broad frequency characteristic of the 
straight magnetostriction hydrophone and its 
sharp directivity above the sonic band are used 
to advantage by the addition of a small super- 


DEVELOPMENT 




Figure 8. Block diagram of JT sonar RLI circuit, 


RLI DC SIGNAL 



114 


SUBMARINE LISTENING EQUIPMENT JP AND JT SYSTEMS 


sonic converter unit. The benefits derived are: 
(1) the ability to pick up enemy signals out- 
side the 14- to 36-kc band of the QB-QC re- 
ceiving equipment or to serve in case that 
equipment became inoperative and (2) the 
ability to discriminate between closely spaced 
targets by listening to the higher frequency 
components of screw noise. 


and bridges across the volume control of the JP 
amplifier. 

Several vernier condensers are required to 
adjust the oscillators to the correct frequency. 
These oscillators utilize a resistance-stabilized 
circuit and vary but slightly in frequency for 
changes in line voltage. The band-pass filter is 
inductance-tuned at the factory and usually re- 


HYDROPHONE 



Figure 9. Block diagram of supersonic converter components. 


A supersonic converter, or converter-ampli- 
fier, provides for converting any 5-kc band in 
the 8- to 65-kc region to the audible band 0.1 to 
5 kc. A block diagram of the circuit is shown 
in Figure 9. Signals from the hydrophone are 
transformer-coupled to a single-stage amplifier 
terminating in a 71-kc low-pass filter. This 
filter reduces any extraneous signals resulting 
from stray coupling of the input stage to the 
two heterodyne oscillators. A three-step at- 
tenuator provides a means of reducing the level 
of echo-ranging signals to prevent overloading 
the grid of the first mixer tube. The first mixer 
is coupled to a heterodyne oscillator which may 
be tuned from 102 to 154 kc by means of a con- 
denser coupled to the panel tuning dial. The 
output of the first mixer is terminated in a 
high-impedance band-pass filter which accepts 
the heterodyned signals in the 89-kc to 94-kc 
pass band. These signals are connected to the 
input of a second mixer stage which is modu- 
lated by a 94-kc fixed oscillator. The 89-kc to 
94-kc signals, therefore, are heterodyned down 
to a 0-5-kc band in the second mixer. The out- 
put of this mixer stage is terminated in a 5-kc 
low-pass filter which also attenuates extraneous 
signals. This filter is terminated in 10,000 ohms 


quires no further adjustment. All five tubes in 
the circuit are of the 6SJ7 type, which simpli- 
fies tube replacement. The heater current of 
these tubes is controlled by a regulator tube to 
reduce the effect of variations in line voltage. 

Power Train, Drive, and Bearing Repeater 
System 

The development of these components in- 
volved the selection of training equipment and 
the detailed design of drive and bearing re- 
peater facilities, which could be installed con- 
veniently in approximately 200 submarines. 
Numerous surveys, layouts, and discussions 
were necessary in order to anticipate the prob- 
lems imposed by a variety of forward torpedo- 
room arrangements. 

Training Gear. Three designs of JP train- 
ing gear in use were designated as Old Ports- 
mouth, New Portsmouth, and Mare Island 
types. Because of the high torque required at 
depths greater than 100 feet, replacement of 
the Old Portsmouth type was undertaken early, 
and action was later taken to standardize the 
Mare Island type for all new boats. 

The proposed use of the 5-foot hydrophone 


DEVELOPMENT 


115 


assembly made it necessary to consider the 
ability of the JP training shafts to withstand 
larger forces. Calculations and tests were made 
of the probable water drag, overturning mo- 
ment, bearing loads, torsional moment, and de- 
flection in the shaft at speeds up to 9 knots. 
These indicated that the deck structure and 
shafts, while not ideal, were satisfactory. These 
studies also enabled the power requirements to 
be established. On the basis of a maximum 
training speed of 5 rpm, a maximum training 
effort equivalent to 6 pounds applied at the 
handwheel through the standard 11-to-l gear- 
box, and a reserve power of over 500 per cent 
for contingencies, a i4-hp drive motor was 
selected. 

One experimental training system having 
slewing-type control was tried out, but experi- 
ence soon proved that a follow-up type control 
in which one revolution of the handwheel would 
rotate the hydrophone approximately 10 de- 
grees was preferable. Several types of train- 
ing systems, including thyratron, hydraulic, 
and amplidyne, were considered from the stand- 
points of overall performance, development 
time required, space, inherent noise, and avail- 
ability. An amplidyne system was chosen in 
which direction and speed of rotation are re- 
motely controlled by means of small manually 
operated handwheels coupled to 5CT synchros. 
A simplified schematic of the elements is shown 
in Figure 10. 

The training motor is a gear-head d-c motor 
rated at hp at 155 rpm. A follow-up synchro 
(5G) is mounted by means of a bracket from 
the motor end bell. 

Power from the gear-head motor is trans- 
mitted to the JP gearbox by means of sheaves 
and two V belts. A hinged handle on the large 
sheave of the training gear shaft makes manual 
training possible. Such a belt drive simplifies 
installation problems and reduces mechanical 
vibration transmitted to the hydrophone shaft. 
This drive rotates the hydrophone at any speed 
up to 4.5 rpm, depending upon the rate of turn- 
ing the handwheel. One revolution of the hand- 
wheel trains the hydrophone approximately 10 
degrees. 

Bearing Repeater System. The bearing re- 
peater equipment, a schematic diagram of 
which is shown in Figure 11, transmits the 


relative bearing of the JT hydrophone to the 
repeaters associated with the target designa- 
tion system [TDS], the torpedo data computer 
[TDC], QB, and QC-JK portions of the WCA- 
type equipment in the conning tower, and to 
the repeater in the JT console. A size 6G 
synchro, geared in a 1-to-l ratio with the hydro- 
phone shaft supplies all the one-speed repeaters ; 
a size 5G geared 36-to-l with the shaft supplies 
the 36-speed repeaters. 



Figure 10. Schematic diagram of JT power train- 
ing system. 


JT-gyro switch boxes, mounted on top of the 
QB and QC-JK remote-control units provide a 
means of bearing presentation to the QB and 
QC-JK operators. For example, when the 
switch is in the JT position, relative JT bear- 
ings are indicated by the North indicator on 
the compass card. 

Sonar Talkback System 

To provide communication between the sonar 
operator in the forward torpedo room and the 
attack team in the conning tower, tests of ex- 





116 


SUBMARINE LISTENING EQUIPMENT JP AND JT SYSTEMS 



perimental equipment installed on fleet sub- 
marines indicated the need for: (1) provision 
of direct two-way communication with ability 
to transmit the sonar signals to the conning 
tower when desired, (2) provision of a sepa- 
rate amplifler for the communication system, 


(3) use of the spare battle phone circuit cables 
(XJA) to facilitate installation and, (4) use of 
a 110- to 120-volt d-c power supply to permit 
operation on all boats during evasive maneu- 
vers. It was also considered essential that voice 
communications from the conning tower and 



DEVELOPMENT 


117 


the signal from the sound gear should reach the 
JT operator over the same headphones and that 
the sonar signal should be received continu- 
ously by the operator except when messages are 
being transmitted from the conning tower. 

Intelligibility tests and characteristic curves 
were run on various microphones, amplifiers, 
speakers, and headphones to determine the 
components most desirable for good articula- 
tion. A dynamic Permoflux Type PDR-8 head- 
phone was selected on the basis of response 


sistance-coupled, feedback unit, is installed 
with the sonar gear in the forward torpedo 
room. Screwdriver adjustments regulate vol- 
ume. 

Talkback Control Unit. The talkback con- 
trol unit, also installed in the forward torpedo 
room, contains the power supply switch, fuse, 
pilot light, and jacks for the sonar operator’s 
headset and spare headphones. Switches are 
also incorporated for controlling voice and so- 
nar signal transmission to the conning tower. 



Figure 12. Components of sonar talkback system. 


characteristic and rugged construction to re- 
place the less rugged crystal-type phones used 
with the JP equipment. The same type of re- 
ceiver was also found to be almost ideal for use 
as a microphone. 

Before the JT equipment was completed, a 
talkback system, which could be installed sepa- 
rately (for use with the JP gear and later with 
the JT gear), was designed. The system con- 
sists of four units: sonar talkback amplifier 
(Navy Type CRV-50200), talkback control unit 
(CRV-23461), talkback speaker unit (CRV- 
49590), and headset (CRV-49586). 

Amplifier. The amplifier, a three-stage, re- 


Talkback Speaker Unit. The talkback speaker 
unit, installed in the conning tower, utilizes 
a 6-inch permanent-magnet type speaker with a 
moistureproof cone and contains a jack for 
headphones. A switch is provided in the speaker 
unit to control a relay for reversing the sound 
circuit, making it possible to communicate from 
the conning tower to the sonar operator by using 
the speaker as a microphone. 

Headset. The headset used by the sonar oper- 
ator contains three dynamic headphones, with 
the third unit, supported on a wire frame from 
the headband, being used as a microphone. The 
four units of the talkback system are shown iri 
Figure 12. 




118 


SUBMARINE LISTENING EQUIPMENT JP AND JT SYSTEMS 


10.7 test and calibration 

FACILITIES 

In addition to the development of compo- 
nents of the JT system itself, consideration 
was given to the problem of providing for test- 
ing and adjusting the equipment in the field. 
It was concluded that a portable test set should 
be made available at bases and tenders to en- 
able technicians and field engineers to check 
and maintain the electronic components of the 
JT system at the time of installation and dur- 
ing refit periods. 

The test set finally evolved consists of an at- 
tenuator and phase-shift system and an elec- 
tronic noise source. The voltage for the attenu- 
ation and phase-shift system may be derived 
from either the self-contained noise source or 
from a conventional externally-connected audio 
oscillator. A meter is provided on the panel 
for calibrating the level at the input of the 
attenuator. The output voltages, the phase of 
which may be altered as previously mentioned, 
are taken across 1-ohm resistors. The output 
may be varied in amplitude from —130 db to 
— 40 db below 1 volt in 1-db steps. Each unit 
is provided with a set of cables and termina- 
tions to permit accurate measuring of the gain 
and phase-sensitivity of the RLI system and 
the sensitivity of the JP amplifier and super- 
sonic converter. 

Because the JT system is intended to indi- 
cate target bearings accurately, it is essential 
that the hydrophone and associated equipment 
be aligned with the hull so that sound and 
periscope bearings agree as closely as possible. 
Complete agreement between sound and peri- 
scope bearings is not attainable under all con- 
ditions because of factors such as sound travel 
time, parallax errors, and acoustic shielding 
from the conning tower. It is also highly desir- 
able that the sound operator be able to check 
the performance of the system periodically dur- 
ing war patrol. 

It was believed possible to meet both these 
requirements by providing a sound source on 
the submarine at a fixed known bearing from 
the JT hydrophone. 

To accomplish this, an NL-130^ hydrophone 


d This unit is described in Division 6, Volume 11. 


(used as a projector) is mounted on the radio 
mast and energized by a small electronic noise 
generator capable of producing a broad-band 
signal similar to propeller noise. Tests of this 
equipment on submarines indicate that, at 
speeds below 3 knots, RLI bearings can be ob- 
tained to within ±0.2 degrees on the reference 
source. A production unit of this bearing cali- 
bration equipment is designated as the sonar 
test target and is intended for use with the 
JP, JT, and WFA gear. 

EXPERIMENTAL SYSTEM AND 
LABORATORY PILOT MODEL 

During the JT sonar development program 
the components of the system were thoroughly 
tested at sea both separately and in various 
combinations. An experimental system incor- 
porating most of the major components of the 
final JT model but utilizing a split magneto- 
striction hydrophone consisting of two 18-inch 
wood-core elements, was used in an extensive 
test program. This system, first bench-tested 
in the laboratory and sea-tested on a surface 
vessel, was subsequently installed on a subma- 
rine for exhaustive tests of bearing accuracy 
and evaluation of its tactical effectiveness. 

During these tests, also, investigation was 
made of a system for automatic control of the 
hydrophone bearing with the RLI signal as a 
control voltage. This feature, known as the 
automatic target folloiver [ATE], was aban- 
doned after preliminary tests because of un- 
satisfactory performance on closely spaced mul- 
tiple targets and because the necessity for 
freezing the system design prohibited further 
development work. 

An indication of the degree of bearing ac- 
curacy attained during tests of the experimen- 
tal system is shown in Figure 13, which records 
the results of a typical check between sonar 
and periscope bearings for an approximately 
circular target vessel course. Other tests of 
this equipment on a submarine indicated that 
it was possible for an operator, after very 
little instruction and practice, to track single 
targets out to 10,000 yards with an accuracy 
of ±0.5 degree. 

A study of the effects on RLI bearings of 
interfering targets of varying intensities and 


PRODUCTION MODflL 


119 


angular relationships with respect to the de- 
sired target indicated that appreciable bearing 
errors are introduced when high-level interfer- 
ing signals are separated from the desired tar- 
get by angles less than that between the on- 
target and secondary zeros of the RLl pattern 


target circling submarine 



(about 17 degrees for the NL-124 hydrophone 
in the band 5 to 9 kc). For this reason, con- 
sideration was given to reducing the RLl lobe 
width by use of a 7- to 12-kc band and a hydro- 
phone having a smaller value of side lobe re- 
duction than the NL-124 unit as discussed 
earlier. It was believed that production might 
be delayed by incorporation of these changes, 
however, and the 5- to 9-kc band and NL-124 
hydrophone were retained. 

MANUFACTURER’S PILOT MODEL 

With the laboratory pilot model as a guide, 
the Radio Corporation of America constructed 
five units of a pilot model preliminary to un- 
dertaking full production of the JT sonar sys- 
tem. Three of these units were installed on 
new-construction submarines at New London, 
Portsmouth, and Mare Island. 

Tests of the manufacturer’s pilot model units 
during the training periods of the submarines 
indicated satisfactory performance, the results 


being about as expected on the basis of the ex- 
perimental system and laboratory pilot model. 
It was established that (1) the RLl is very 
helpful in maintaining accurate bearings on 
single targets, (2) the supersonic converter 
enables the operator to discriminate between 
targets separated by as little as 3 or 4 degrees, 
(3) the power drive enables the operator to 
train the hydrophone accurately and without 
appreciable physical effort and, (4) the sonar 
talkback and bearing repeater systems permit 
close coordination between the sound operator 
and the attack team and enable the QB or QC 
operator to preset the projector bearing to aid 
in obtaining single-ping ranges. 

10.10 PRODUCTION MODEL 

The production units of the JT sonar equip- 
ment closely follow the design of the laboratory 
and manufacturer’s pilot models. The system 
is made up of five major assemblies: master 
control unit, power training and bearing re- 
peater equipment, hydrophone assembly, super- 
sonic converter, and the sonar talkback equip- 
ment. 

The master control unit. Figure 14, includ- 
ing the RLl, the listening components, and 
the control facilities, is the largest unit in the 
JT sonar system. It has a space requirement 
of 19% 6 inches width, I 6 V 2 inches depth, and 
371/2 inches height, including the height of the 
shock mounts, three on either side of the base, 
which isolate the unit from the deck. All com- 
ponents are contained in two metal cabinets 
which are attached and interwired. The lower 
cabinet contains a steel chassis 3 inches deep 
hinged to the cabinet at the bottom edge. The 
larger components, including the tubes, are 
mounted on the rear of this chassis so that the 
wiring, resistors, and similar parts are acces- 
sible from the front to facilitate servicing. The 
lower portion of the chassis is covered with a 
separate panel attached with thumbscrews and 
the upper portion is covered by a hinged panel 
on which are mounted the circuit controls. 

At the top of the unit is the control cabinet 
having a sloping front and a 3-inch extension 
beyond the rear of the lower cabinet to provide 
an entrance for connecting cables to the main 
terminal board mounted inside the rear of this 




120 


SUBMARINE LISTENING EQUIPMENT JP AND JT SYSTEMS 


cabinet. The chassis may be pulled forward 
on slides giving access to the main terminal 
board in the rear and to the components 
mounted on the chassis and panel. 

The power training handwheel, coupled to 
an inertial flywheel and a control transformer 



Figure 14. JT sonar master control unit. 

(5CT), is mounted at the right on the vertical 
portion of the upper chassis beneath the sloping 
panel, and on the left of the chassis is the am- 
plidyne power training amplifier. The RLI 
meter is mounted at the center of the panel 
with the hydrophone relative bearing repeater 
dial just above it in the same window. In a 
separate window below the meter is a recipro- 
cal bearing dial. Internal lighting is provided 
for the meter and dials. On the left side of the 
sloping panel are located the jacks and switches 


of the sonar talkback control unit (see develop- 
ment section). The bearing repeater synchro 
(5F) is mounted in a casting attached to the 
rear of the panel. 

The hydrophone assembly, weighing about 
130 pounds, consists of the hydrophone, baffle, 
and sound absorbing coupler shown assembled 
in Figure 15. 



Figure 15. JT hydrophone assembly. 

The 5-foot hydrophone, bolted to the training 
shaft flange, is made up of five pairs of per- 
manent - magnet type cylindrical hydrophone 
elements 5% inches in length and 1% inches 
in diameter assembled coaxially on a rigid 
brass tube. A cylinder of cardboard between 
the brass tube and hydrophone units provides 
a pressure release to assure good acoustical 
performance. The hydrophone is described as 
a delobed type, since each pair of units on op- 
posite sides of the center is reduced in sensi- 
tivity by reducing the number of turns suc- 
cessively from 245 for the center pair to 95 
turns for each end unit. The assembly of 
hydrophone elements is encased in a 21 / 2 -inch 
diameter i/^-inch thick neoprene cylinder and 
impregnated with a Resinox compound. A 
polystyrene connection jack and gland seat is 
inserted in one end to connect with a four-wire 
plug and a four-conductor shielded hydrophone 
cable. A gland assembly seals the cable en- 
trance and facilitates replacement of the hydro- 
phone after initial installation. 

The baffle is a streamlined, stainless-steel 
fairing 641/2 inches long, 4 inches high, and 
9% inches wide. It provides a mount for the 
hydrophone and a form for attaching a %-inch 
Cell-tite rubber blanket which reduces the back- 
sensitivity of the hydrophone 18 to 20 db in the 
5- to 9-kc frequency band. 


O 



PERFORMANCE 


121 


The coupler consists of 2 brass disks 10 inches 
in diameter and 1 inch thick, bonded together 
with a 1-inch layer of 25 to 30-durometer rub- 
ber. Precautions are taken to obtain a strong 
durable bond which is not susceptible to chan- 
ging climatic conditions or the deteriorating 
action of salt water. Three rubber-insulated 
through-bolts safeguard against loss of the hy- 
drophone if the rubber bond fails. The baffle is 
secured to the top of the coupler. 


PERFORMANCE 

Tests of the experimental JT models as well 
as several manufacturer’s pilot models showed 
that the improvements over the JP gear add 
to the ease of operation and the speed with 
which data can be transmitted. Corroborating 
patrol reports which indicate performance un- 
der actual battle conditions are not yet avail- 
able. 


Chapter 11 


THE 692 SONAR SYSTEM 



The 692 sonar, designed by Bell Telephone Laboratories, is a multipurpose experimental 
system designed for submarine use. Utilizing the WF A three-section crystal projector, it pro- 
vides for listening over the band 200 c to 60 kc with bearing deviation indication for frequen- 
cies between 15 kc to 60 kc. Other features include maintenance of true bearing [MT5], auto- 
matic target following, automatic torpedo detection, arid self-noise monitoring. It also proposes 
QBF-type echo-ranging equipment to operate over the range 10 kc to 50 kc and modified to provide 
short-pulse mine detection with plan position indicator [PPI] presentation. Although the devel- 
opment program ivas not concluded, preliminary operational tests of the listening system showed 
good sensitivity and bearing accuracy. 


Ill INTRODUCTION 

4T THE TIME WORK was begun on the 692 
sonar system, the standard systems in use 
on submarines were rather limited in scope. 
They consisted for the most part of a JK-QC 
projector combination for listening and echo 
ranging over a fairly narrow band of frequen- 
cies in the region of 25 kc. This was supple- 
mented by the JP system, which was entirely 
separate and covered the audible range. Later 
adaptations extended the JP into the supersonic 
range. The training mechanism used with the 


JK-QC projector did not permit the rapid scan- 
ning needed to identify low-intensity signals. 
The JP was trained manually and could differ- 
entiate signals but required considerable phy- 
sical effort. None of these systems employed 
visual bearing indicators of the phase-sensitive 
type. The echo-ranging available with the QC 
projector was the standard 25-kc long pulse 
type used in antisubmarine work. 

Looking toward improved submarine sonar 
systems, the outstanding need was for a system 
which would supply information to the subma- 
rine commander operating below periscope 



122 


G 


INTRODUCTION 


123 


depth, comparable to what he can obtain with 
his periscope. The best possible sonar cannot 
equal the capabilities of periscope and radar. 
Low-frequency sonar may excel in range but 
can not compare in accuracy. High-frequency 
sonar can provide good accuracy but falls short 
in range. Since the important factor in deep 
operations is security, echo-ranging of any kind 
is not the final answer. Triangulation ranging 
on sound bearings is a possibility. Sound bear- 
ing accuracy then becomes of primary impor- 
tance and a requirement of ±0.1 degree is not 
unreasonable. This in turn imposes certain re- 
quirements on the training system as well as 
the projector. A complete sonar system should 
also include means for scanning mine fields, 
self-noise monitoring, torpedo detection, loca- 
tion of depth charges, sonic depth finding, and 
underwater communication. 

The equipment designated 692 submarine 
sonar, from the OSRD contract number, was 
designed to facilitate the investigation of sonar 
requirements rather than to supply a working 
system. Its scope, therefore, is quite broad as 
regards component features, controls, and ad- 
justments, and its parts are not completely in- 
tegrated as they would be in a standard equip- 
ment.® 

The original project called for the develop- 
ment of a listening system only, which was to 
be supplemented with a standard surface vessel 
type of echo-ranging equipment (QJB) oper- 
ating at 24 kc and at 50 kc. Subsequently the 
project was enlarged to include the develop- 
ment of a short-pulse, high-peak power, echo- 
ranging equipment to operate over the range of 
frequencies from 10 kc to 50 kc. Finally, it was 
agreed to include in the echo-ranging system a 
plan position indicator [PPI] for mine detec- 
tion.** 

The initial requirements included continuous 

® In order to include the 692 sonar system in this 
volume, it was necessary to confine this material to a 
generalized description of the system, omitting actual 
circuit details and operation. These generally follow 
component circuits described in this volume and Divi- 
sion 6, Volume 15. Complete information on actual 
circuits and their performance under experimental con- 
ditions may be found in reference 1, which has been 
microfilmed. 

Details of the echo-ranging equipment embodied in 
the 692 sonar are discussed separately in Division 6, 
Volume 15. 


search at speeds up to 60 rpm with an indicator 
preferably not of the cathode-ray oscilloscope 
[CRO] type, rapid shifting between continuous 
search, and hand training, the latter to be 
accurate within ± 0.5 degree or better, using 
a phase-sensitive bearing indicator, automatic 
or aided target tracking, and maintenance of 
true hearing [MTB].| The self-noise of the 
training system was to be low enough so as not 
to affect listening. The listening system had to 
be capable of differentiating between two tar- 
gets of the same intensity when they are 5 
degrees or more apart. The frequency range 
was to be from 15 to 50 kc, with provision for 
listening to suitable bandwidths with both loud- 
speaker and headphones. 

The completed system, which has been de- 
livered to the U. S. Navy at New London, Con- 
necticut, contains all the features outlined 
above. It includes a three-section projector 
which is a prototype of the projector used with 
the WFA sonar. This is one of several features 
which the two systems have in common and 
which were derived from the early development 
work on Contract No. 692. The indicator for 
the listening system displays the location of 
targets by means of an azimuth circle of lights 
and has proved adequate for tracking torpe- 
does. The MTB feature operates satisfactorily 
either with a step-by-step or synchro type of 
compass repeater. Either continuous search lis- 
tening [CSL], automatic tracking [AUTO], or 
hand training [HAND] may be rapidly se- 
lected on one switch. 

Measurements of the bearing accuracy of the 
automatic target tracking system at sea indi- 
cated a 0.15-degree standard deviation of sound 
bearings with respect to the stern of a target 
ship. This means that the bearing accuracy of 
the 692 sonar will be well within the ± 0.5- 
degree requirement. The self-noise of the train- 
ing system is well below the lowest ambient 
noise levels at distances greater than 10 yards 
from the projector. The self-noise picked up by 
the projector is below the internal noise in the 
system except at frequencies below 1,000 c and 
at speeds in excess of 20 degrees per second. 
The internal noise is below the lowest ambient 
noise levels in the intermediate range of fre- 
quencies and only slightly above at the ends of 
the frequency range. The useful frequencies 


124 


THE 692 SONAR SYSTEM 



oa A on 



Figure 2. Block diagram of listening system 




DESCRIPTION OF EQUIPMENT 


125 


extend from about 200 c to 60 kc, but the band 
below 10 kc is used only for detection listening 
and not for bearing determination, because of 
its poor directivity. 

The ability of the system to differentiate be- 
tween targets depends upon the beam width of 
the projector. By avoiding the use of side lobe 
reduction which broadens the beam, the dis- 
crimination at frequencies above 50 kc is suf- 
ficient to separate two targets of equal signal 
strength 5 degrees apart. This was confirmed 
experimentally. 

Although the field trials of the 692 sonar 
were of a limited nature, sufficient data were 
obtained to confirm the above statements on 
performance. No trials were made of the short- 
pulse echo-ranging equipment except to check 
its operation. It is expected that further trials 
will be made by the Navy to obtain additional 
information on the capabilities of the system 
as a whole. 

2 DESCRIPTION OF EQUIPMENT 

The four major components of the 692 sonar 
are the projector, the projector training sys- 
tem, the listening stack, and the short-pulse 
echo-ranging stack. The listening system is 
itself composed of a number of units, each of 
which is described separately. A block diagram 
of the listening system is shown in Figure 2. 


Projector 

The design requirements called for a pro- 
jector to cover the range from 200 c to 60 kc 
for listening and from 10 to 60 kc for echo 
ranging. Also, the unit should not be more 
than 36 inches high and should be in a cylin- 
drical case not more than 13 inches in diam- 
eter. As 13-inch tubing was not commercially 
available, the latter requirement was revised 
to 14 inches. A requirement that the unit be 
capable of withstanding gun blast and a static 
pressure of at least 400 psi was also specified. 

By specifying both size and frequency range, 
the more important characteristics of the pro- 
jector, including its directivity, were defined. 
It was agreed that the requirement could best 
be met at the time by employing a three-section 


projector; one section to cover the 1-f range 
from 200 c to 15 kc; a section consisting of a 
modified QBF projector to cover the inter- 
mediate frequencies from 15 to 30 kc; and a 
h-f crystal plate of the same width but half 
the height of the QBF plate to cover the range 
from 30 to 60 kc. The 1-f unit consists of a line 
of four diaphragm-type'^ hydrophones, each con- 
taining a single block of crystals of the type 
used in the QBF plate. These four units are 
mounted between the h-f and i-f sections. The 
h-f unit is mounted at the top in order to mini- 
mize diffraction about obstacles on the deck of 
the submarine. The installed projector is shown 
in Figure 3. 



Figure 3. The 692 submarine sonar projector 
mounted on the deck of a submarine. 


The h-f unit was made the same width as the 
other units in order to obtain maximum hori- 
zontal directivity. However, its height was 
made only one-half the width, so that the verti- 
cal directivity pattern would not be too sharp. 
Too narrow a beam in the vertical plane would 
unduly restrict the area from which signals 
could be received. Although none of the units 
in the 22Z-1 projector was tapered in the hori- 


126 


THE 692 SONAR SYSTEM 


zontal plane, both the h-f and i-f units were 
tapered in the vertical plane to reduce the effect 
of reverberation. 

In addition to meeting the Navy require- 
ments, the design was aimed at keeping the 
absolute efficiency as high as possible over the 
listening range, thereby keeping the thermal 
noise low with respect to ambient water noise. 
This is particularly important at the higher 
frequencies. An effort was also made to keep 
the phase shift between the halves of the pro- 
jector to a minimum in order to provide good 
balance for the phase-sensitive detector. 

Crystal Arrays 

The arrays are mounted on a steel frame 
which bolts to the housing. Front and back 
views of this assembly are shown in Figures 



Figure 4. Front view of crystal arrays in the 692 
projector. 


4 and 5. The h-f array is at the bottom of the 
photograph, the sonic or 1-f array is in the 
center and the i-f array from 10 to 30 kc is at 
the top. When mounted topside on a submarine, 
this order is reversed and the i-f array is at 
the bottom. 



Figure 5. Rear view of crystal arrays in the 692 
projector. 

As shown in Figure 5, the h-f and i-f arrays 
are supported at the four corners by brackets 
which are mounted in a shock-insulating type 
of rubber support developed for the QBF pro- 
jector. The 1-f array is similarly shock-mounted 
to a cross member of the frame. The frame also 
supports terminal strips, a wiring form, re- 
peating coils for the h-f and 1-f arrays and 


DESCRIPTION OF EQUIPMENT 


127 


protective neon lamps. These are not shown in 
the photographs. 

Projector Characteristics 
The characteristics of the projector to a large 
extent determine the capabilities of any sonar 
system. No refinement of the electric circuit 


the water are shown in Figure 7. The h-f unit 
is somewhat more efficient in this respect, due 
primarily to a better directivity index. 

Internal Noise. The internal noise spectra for 
the projector are shown in Figure 8. The 1-f 
unit is poorest in this respect. A higher level 
of internal noise can be tolerated at the low 



300 500 700 1000 2000 3000 5000 7000 10,000 20,000 30,000 50,000 70,000 100,000 > 

FREQUENCY IN CYCLES PER SECOND 

Figure 6. Open circuit calibration of WFA No. 22-Z projector used as a hydrophone. 


can make up for deficiencies in the projector, 
particularly as regards its internal noise thresh- 
old and its directivity. Frequency response is 
not basically important but may be used to 
calibrate the output in various listening bands 
in terms of sound pressure in the water. 

Frequency Response. The open-circuit cali- 
bration of the projector used as a hydrophone 
is shown in Figure 6. This type of measurement 
is significant from a design standpoint. It 
shows a high peak at the cutoff frequency of 
the transformer which is compensated for in 
the amplifier input transformer. 

The calibration of the h-f and i-f sections of 
the projector when used to transmit sound into 



FREQUENCY IN CYCLES 

Figure 7. Calibration of h-f and i-f sections of 
WFA No. 22-Z projector. 


frequency because the spectrum of ambient and 
self-noise rises toward the low end. Further- 
more, the directivity becomes less and thus a 



128 


THE 692 SONAR SYSTEM 


higher level of ambient noise is accepted. How- 
ever the 1-f unit is limited by internal noise 
wherever low ambient and self-noise levels 
prevail. 


and 36 kc are given in Figures 9B, 9C, 9D, 9E, 
and 9F, respectively. With the exception of the 
pattern for 36 kc, these curves show increase 
in directivity with frequency, which is in agree- 



Figure 8. Internal noise spectra of WFA No. 22-Z projector used in the 692 submarine sonar. 


Directivity. The calculated directivity pat- 
terns for the i-f array at 24.5 kc in both the 
horizontal and vertical planes are shown in 
Figure 9A. These patterns show the effect of 
the taper, which reduces the side lobes at the 
expense of the main lobe, which becomes larger. 
The patterns also show that the directivity of 
the individual blocks has a slight effect on the 
side lobes. 

An array with taper in the horizontal plane 
is considered desirable from an echo-ranging 
standpoint, as the reduced side lobes minimize 
the possibility of errors due to false echoes. 
However, when listening with a phase actuated 
locator [PAL], such an array is not so desir- 
able from an interference standpoint as a linear 
array. This is shown by the curves in Figure 
10, where the bearing error in degrees is 
plotted against angular separation between the 
target and an interfering signal of equal inten- 
sity for both tapered and linear arrays. It can 
be seen that the bearing error when using a 
tapered array is greater than that for a linear 
array when the angular separation between 
target and interference is between 9.5 and 17.5 
degrees. 

The measured directivity patterns of the i-f 
array in the horizontal plane at 12, 18, 24, 30, 


ment with the computed variation of directivity 
index shown with frequency in Figure 11. The 
measured pattern at 24 kc. Figure 9D, com- 
pares favorably with the calculated pattern in 
Figure 9A for 24.5 kc. The pattern at 36 kc is 
typical of what happens to the directivity out- 
side the useful range where the response falls 
off and the phase conditions over the face of the 
projector begin to vary. 

The calculated horizontal and vertical direc- 
tivity patterns at 49 kc for the h-f array are 
shown in Figure 12A. The measured horizontal 
directivity patterns for this array at 30, 40, 50, 
60, and 80 kc are shown in Figures 12B, 12C, 
12D, 12E, and 12F. 

The 1-f array in the 22Z-1 projector has rela- 
tively low directivity, due to dimensional re- 
strictions. It is essentially nondirective in a 
vertical plane and its measured directivity in a 
horizontal plane for 2.2, 6, 9, and 15 kc is 
shown in Figure 13. It will be observed that 
at 2.2 kc there is practically no directivity and 
at 15 kc there is only moderate directivity. Be- 
cause of its poor directivity, this array is used 
for listening only and no attempt is made to use 
the PAL with it because of the inability to dis- 
tinguish among sound sources. 




DESCRIPTION OF EQUIPMENT 


129 


HORIZONTAL PATTEI 
NO TAPER 

/ \ 

VERTICAL PATTERNi 
, 2, 3,5, 6, TAPER 


300 


270 



300 


270 


240 


180 


120 



240 


180 


120 


330 


30 


330 


30 




60 


90 


300 


270 


240 



WATER TEMPERATURE = 56 F . 
TEST DISTANCE = 12.5 FEET 


300 


90 300 


120 240 



210 


180 


150 


210 


180 


150 


Figure 9. Directivity patterns for i-f unit of WFA No. 22-Z: (A) Calculated vertical and horizontal patterns 
at 24.5 kc in db down from maximum response; (B) — (F) Measured horizontal patterns at several frequen- 
cies, in db down from maximum response. 








130 


THE 692 SONAR SYSTEM 



ANGULAR SEPARATION BETWEEN TARGET AND SOURCE OF INTERFERENCE IN DEGREES 
Figure 10. Error of phase-actuated target bearing indication caused by an interfering signal of equal intensity. 

ing sonar systems to meet the more stringent 
requirements discussed below. 


Performance Requirements 

Several requirements were initially empha- 
sized. First it was considered desirable to be 
able to align positively the acoustic axis of the 
projector within ±0.1 degree of any bearing 
desired. For following targets, a smooth con- 
tinuous shaft speed range adjustment was 
sought from about 0.1 degree per second for 
slow ships at long ranges to about 4 degrees 
per second for fast ships at close ranges. 
Higher shaft speeds up to about 30 degrees per 
second were considered necessary for slewing 
quickly from one bearing position to another 
and a single high speed of about 360 degrees 
per second was needed for continuous search. 
This wide range of shaft speed had to be ac- 


0 

CD 

O 


O 

z 

--20 
> 

tS-30 

UJ 

Q 0.2 I 2 3 5 7 10 20 30 50 70 100 

FREQUENCY IN KC 

Figure 11. Computed variation of directivity index 
with frequency for the WFA No. 22-Z projector. 


11.2.2 Projector Training System 

The methods employed in training the pro- 
jector have a direct bearing on the performance 
of the entire sonar system. To provide the 
bearing accuracy and speed range sought for 
in this training system, it was necessary to 
employ methods other than those used in exist- 


LOW FREQUEN( 
UNIT 












ir 

HEF 

REQl 

^MEC 

HA1 

FE 







FI 

JENCY UNIT 

1 1 i 

HIGH FREQl 

JENI 

kJ 

CY Ul 

NIT 














DESCRIPTION OF EQUIPMENT 


131 







210 “ 


180 


150 “ 


210 “ 


180 


150 “ 


Figure 12. Directivity patterns for h-f unit of WFA No. 22-Z: (A) Calculated vertical and horizontal pat- 
terns at 49 kc; (B) — (F) Measured horizontal patterns at several frequencies. 












132 


THE 692 SONAR SYSTEM 


330 “ 0 “ 30 “ 





Figure 13. Measured directivity patterns for 1-f unit of WFA No. 22-Z in the horizontal plane at several frequencies. 


DESCRIPTION OF EQUIPMENT 


133 


complished» without discontinuity, except be- 
tween the slewing and search speeds, by a 
mechanism that could be controlled from a 
remote position and which would neither intro- 
duce noise into the listening system nor radi- 
ate sufficient sound into the water to endanger 
the security of the ship. The noise problem was 
considered of prime importance and much at- 
tention was given to design features that would 
be expected to reduce the mechanical vibration 
transmitted to the training shaft or to the 
ship's hull. Another characteristic of the train- 
ing system that it was necessary to consider 
at the outset was its adaptability to some form 
of automatic target tracking arrangement. 

Speed Control System 

After considering many possible arrange- 
ments to cover the wide speed range, including 
systems with two or three motors, magnetic 
and indexing clutches for selecting different 
gear ratios, mechanisms outside the hull sealed 
in oil, and differential and planetary gear sys- 
tems, there was evolved a drive system of 
attractive mechanical simplicity operating 
through the hull with one motor. This design 
was made possible by a wide-range motor speed 
control system using mercury contact relays. 

Training Mechanism 

In order to couple the motor to the training 
shaft, it was necessary to design and build a 
mechanism assembly with a suitable speed- 
reducing gear and to provide a take-off drive 
for the coarse and fine synchro generators used 
for remote bearing indication. The problems in- 
volved were unusual because of the high accu- 
racy of training sought, the wide range of 
speed required, the probability of bearing and 
gear noise, particularly at the search speed, 
and the necessity of avoiding resonances and 
excessive displacements in the supporting mem- 
bers. Flexible supports are necessary to isolate 
the training shaft from undesirable vibrations 
but excessive displacements in these suspen- 
sions would affect the positional accuracy of the 
training mechanism. Furthermore, the suspen- 
sions and shaft couplings are important ele- 


ments in the feedback loop of any automatic 
target tracking system working through the 
mechanism, therefore they were designed with 
mechanical impedance characteristics to keep 
to a minimum any phase or amplitude effects 
at frequencies below 10 or 15 c. 

The gear system for coupling the drive motor 
to the training shaft irequired features that 
were not obtainable in commercial gear boxes. 
The training accuracy desired for the system 
required that backlash and run-out errors in 
the gears be less than 0.1 degree, which is 
better than could be assured by any commercial 
supplier. Also, it was particularly important 
that gear noise be very low. 

The gear system designed and built consists 
of two worm gears mounted on a hub through 
which the training shaft can be passed. The 
worm shaft is carried by ball bearings in an 
eccentric bushing to permit a close and precise 
adjustment of the mesh between the worm and 
gear. Both worm gears and their mating gears 
were specially cut to order with the highest 
precision equipment available, and the whole 
gear system is enclosed in an oil-filled housing. 

To attenuate any residual gear and bearing 
noise that might be transmitted to the training 
shaft, a special, flexible coupling was designed 
for use between the gear hub and the training 
shaft. This type of coupling introduces a high 
compliance to all translational forces between 
the gear box and the training shaft and thereby 
attenuates noise vibrations having translational 
motions. However for torsional motions this 
coupling is very rigid. Measurements were 
made which showed the torsional displacement 
to be considerably less than 0.1 degree when 
transmitting a full load torque of about 30 
foot-pounds. 

The drive motor is, in reality, two separate 
motors in one housing, as may be seen in the 
schematic diagram. Figure 14. These have two 
separate dipole fields with their pole axes at 90 
degrees to reduce intercoupling and two sepa- 
rate armatures and commutators, one for 
low-speed operation and the other for high- 
speed operation or back emf generation. Both 
armatures have winding slots spiraled by one 
slot in pitch to reduce cogging. The brushes and 
holding frame were designed to give quiet oper- 




134 


THE 692 SONAR SYSTEM 


ation and good commutation for either direc- 
tion of armature rotation. 

Listening System 

The listening equipment in the 692 sonar 
covers a broad frequency range with means for 
selecting bands of different widths anywhere 


The i-f and h-f units pass through a selector 
switch on the control panel. From there each 
half of the projector goes through its own 
preamplifier and then through a mixer circuit 
which leaves the sum on the right channel and 
the difference on the left channel. These are 
amplified in accordance with the setting of the 
gain control. The outputs then go to the modu- 


100-140 VOLTS DC 


left^aIa, right 
'VYV ^ 




25 

VOLTS 

60 

CYCLES 

O 

O 



Figure 14. Schematic of training control circuit, 692 submarine sonar. 


in the range. A block schematic of the system 
is shown in Figure 2. This also shows the train- 
ing controls which form part of the listening 
stack. The various panels, as well as their com- 
ponents, are blocked off. 

The 1-f unit of the projector, which is not 
involved in echo ranging, is carried through the 
slip rings and junction box directly to its own 
preamplifier in the measuring panel. Contacts 
on the band selector switch on this panel, when 
set on band 1, carry the output of the amplifier 
directly to a 5-kc low-pass filter and thence to 
the listening amplifier. When the selector 
switch is on band 2 the output of the preampli- 
fier is taken to a modulator where it is put on 
a 190-kc carrier. From there it passes through 
a filter which selects a 10-kc band around 
200 kc. This is heterodyned at 200 kc in the 
demodulator, resulting in a folded band where 
200 kc becomes zero frequency and both 195 
and 205 kc become 5 kc. This is then passed 
through the 5-kc low-pass filter and on to the 
listening amplifier. 


lator panel, where both the sum and difference 
are put on a carrier whose frequency is selected 
by the mid-band dial marked from 10 to 60 kc. 
This dial actually controls a variable oscillator 
working between 145 kc and 191 kc. For in- 
stance, if a 4-kc band from 10 to 14 kc is 
desired, the mid-band dial is set at 12 kc, the 
signal from the i-f projector would then modu- 
late 191 kc, resulting in a sum output from 
201 to 205 kc. Similarly, if it were desired to 
use 56 to 60 kc, the mid-band would be set at 
58 kc, which corresponds to 145 kc, again re- 
sulting in an output from 201 to 205 kc. The 
output from the sum channel is tapped before 
it reaches the 4-kc wide filter and brought up 
to the sound level panel where it is available 
for band 3. This is similar to band 2 and makes 
available a 10-kc folded band from the i-f and 
h-f units. 

The output also passes through a 4-kc wide 
filter to the demodulator where it is hetero- 
dyned at 200 kc. At this point a 90-degree 
phase shift is introduced between the sum and 



DESCRIPTION OF EQUIPMENT 


135 


difference channels. The demodulated output of 
the sum channel, which consists of a band from 
1 to 5 kc, is fed directly to the listening ampli- 
fier. The same band, together with the corre- 
sponding one from the difference channel, goes 
to a phase-sensitive detector. 

A novel use is made of the voltages which 
operate to control the volume in the detector 
automatically. Since these voltages follow the 
levels in the two channels, their combination 
forms a sum which is more sensitive to phase 
than the direct combination of the two halves 
of the projector which is used for listening. 
This output furnishes the signal to control in- 
dicator lights on continuous search. The nor- 
mal output of the phase - sensitive detector 
which is zero for the projector “on bearing” 
is used to operate the meter associated with 
the vernier scale in the indicator panel. It is 
also fed to a d-c amplifier in the detector panel 
and thence through the MTB panel, the AUTO 
switch and servoamplifier on the control panel, 
and from there to the motor control unit. This 
is a servo loop whereby the projector output 
generates a voltage proportional to the angle 
the projector makes with the target, and this 
voltage is fed back to the training motor which 
rotates the projector until it is on bearing and 
no longer generates the voltage. 

The MTB panel serves as a junction for the 
output of the d-c amplifier in the detector and 
the input to the servoamplifier. However, the 
MTB circuit may be switched to this input in 
order to control the bearing of the projector. 
Even when not controlling, it shows the true 
bearing of the projector by combining the in- 
formation received from the synchro generator 
at the projector shaft with that from the ship’s 
gyro compass. 

The listening equipment is housed in a stack, 
shown in Figure 15, consisting of six cabinets. 
The complete assembly weighs about 1,000 
pounds. In order to reduce the weight, the 
cabinets and panels were made of suitable alu- 
minum alloys. When mounted on shipboard, 
the cabinets are bolted together and are fas- 
tened at the top and bottom through rubber 
shock mountings. 

The stack is arranged so that the indicator 
panel is somewhat below the average eye level 
when the operator is standing and is somewhat 


above when sitting. The controls are grouped 
on the top four panels within reach of an oper- 
ator either standing or sitting. Each cabinet 
is described in detail in the following sections. 



Figure 15. Stack housing listening equipment, 692 
submarine sonar. 



136 


THE 692 SONAR SYSTEM 


Sound Level Panel 

The topmost cabinet of the listening stack 
contains the audio listening amplifiers and a 
sound level indicator. It also contains some 
modulators, an alarm generator, and a loud- 
speaker. As shown in Figure 16, the level meter 



Figure 16. Close-up of top cabinet of listening 

stack, showing sound level meter. 

appears at the upper left. The listening inputs 
are controlled by the left-hand dial. The first two 
steps connect the 1-f array to the listening sys- 
tem. The first step takes the output of the 1-f 
preamplifier directly through a 5-kc low-pass 
filter and thence to the loudspeaker or head- 
phones through the listening amplifier. The sec- 
ond step takes it to a modulator in such a way 
that the 5-kc bands either side of 10 kc are 
superimposed and stepped down to the range 
below 5 kc before passing through the filter. 
The third step does the same for a 10-kc band 
centered at 3 kc below the mid-band frequency 
selected on the modulator panel. In this case 
either the h-f or i-f element of the projector is 
connected to the system, as determined by a 
switch on the control panel described later. 

Indicator Panel 

The indicator panel shown in Figure 17 fur- 
nishes the bearing information obtained from 
the listening equipment. The outer azimuth 
circle is for relative bearings. Just within this 
is a ring of diamond-shaped light apertures. A 
white pointer travels within this circle to in- 
dicate the position of the projector. There then 
follows a compass card which is connected 
through a servomechanism to the ship’s gyro 


compass and shows the true bearing. The 
hearing deviation indicator [BDI] and the ver- 
nier relative-bearing dial appear in an opening 
cut in the central mask. 

As the projector is rotated, the white pointer 
rotates with it. When the projector is brought 
to bear on a target the position of pointer on 
the outer circle indicates the relative bearing 
of the target with an accuracy of a few degrees. 
The BDI meter needle swings from side to side 
as the projector passes through the target and 
is at dead center when the projector is exactly 
on bearing. The exact bearing can then be read 
from the small dial to the nearest tenth of a 
degree. For instance, the target bearing indi- 
cated on the photograph is 343.2 degrees. If 
it is desired to know the true bearing, this can 
be read to the nearest degree on the compass 
card. There is no relation between this and 
the small dial. When the listening equipment 
is switched over to continuous search, a light 



Figure 17. Close-up of second cabinet of listening 
stack, showing indicator panel. 


flashes at the bearing of a target and there is 
sufficient delay in the circuit to keep the light 
lit between revolutions as long as the target is 
within sound range. 

The upper left-hand dial controls the in- 
tensity of the dial lighting. The large plastic 
dial face is of a color designed to pass those 
rays in the red portion of the spectrum 
(greater than 6,000 Angstroms) which have 


DESCRIPTION OF EQUIPMENT 


137 


been found to have the least effect on night 
vision. The upper right-hand dial is used to 
synchronize the servomotor driving the com- 
pass card with the ship’s gyro. This is required 
only when the ship’s compass system is of the 
step-by-step type. 

The lower left-hand dial controls the sensi- 
tivity of the BDI meter. This is optional with 
the operator, depending on how much meter 
swing he prefers when passing through a tar- 
get. The lower right-hand dial controls the 
intensity of the flashing lamps. 

Modulator Panel 

The modulator panel shown in Figure 18 is 
arranged to select the mid-band frequency for 
either the h-f or i-f projector units. The cen- 
ter dial drives the condenser plates of a vari- 
able oscillator and displays the mid-band fre- 
quency for the i-f unit on the left and for the 
h-f unit on the right. The lower dial switches 



Figure 18. Close-up of third cabinet of listening 
stack, the modulator panel, showing frequency se- 
lector dial. 

from a 4-kc bandwidth to the full bandwidth 
of either the h-f or i-f units. The full band- 
width feature was included to provide a means 
for detecting, on continuous search, enemy 
echo ranging which might appear anywhere in 
the band and would be difficult to locate with 
a narrow-band listening system. 

Control Panel 

The i-f and h-f sections of the projector are 
directly connected to the control panel shown 
in Figure 19 and are selected by means of the 


upper left-hand dial. This panel contains the 
initial amplification for these elements, also 
the mixers to obtain the sum and difference 
channels for the BDI, the initial gain control. 



Figure 19. Control, or fourth, cabinet of listening 

stack. 

an additional 35 db of amplification in each 
channel, and the training control servoampli- 
fier. The handwheel may be turned either to 
the left or right within a 180-degree arc, the 
excursion determining the speed in that direc- 
tion at which the projector rotates. In other 
words, for a very small change of bearing, the 
wheel is turned a very small amount to provide 
a slow training rate in the desired direction. 
The action of the projector is of course shown 
by the pointer on the indicator panel. 

Under the handwheel is a switch for select- 
ing one of three conditions : continuous search 
listening [CSL], hand training, and automatic 
tracking. The operator can locate a target by 
hand operation of the wheel and immediately 
switch over to automatic tracking, whereupon 
the projector stays on the target without re- 
quiring any further manual operation. If no 
targets are within sound range, he may wish 
to set the projector in motion by switching to 
CSL, whereupon it continues to rotate and 
flashes one or more lights at the bearing of a 
target when it comes within range. The center 
dial marked “level” controls the gain of the 
panel in 5-db steps. 

Detector Panel 

The detector panel shown in Figure 20 am- 
plifies the sum and difference outputs of the 
modulator panel, using either a 4-kc band re- 


138 


THE 692 SONAR SYSTEM 


duced to the audible range or the full band 
of the i-f and h-f projector units. The phase 
difference between the two channels using a 



Figure 20. Detector, or next-to-bottom, cabinet of 

listening stack. 

4-kc band provides the BDI indication of bear- 
ing. The circuit also provides the equivalent 
of a phase-sensitive maximum output which, 
when amplified and limited, provides the signal 
to flash the proper lamp on continuous search. 
The BDI voltage is applied to the meter and 
is also amplified and limited to furnish a con- 
trol voltage which is fed back to the d-c servo- 
amplifier for automatic tracking. A test tone 
generator for adjusting the BDI is also 
mounted on this panel. The ADJ zero dial is 
used to center the meter during the test op- 
eration. 

Power Supply Panel 

This panel contains two regulated plate sup- 
plies, each of nominal 300 volts. The load has 
been divided between them in such a way as 
to avoid noise troubles. The combined return 
is used to heat the cathodes of four tubes in 
critical portions of the listening system circuit. 
Pin jacks are mounted on the panel for con- 
necting a voltmeter to check each supply. 

Maintenance of True Bearing [MTB] 
Panel 

This panel, shown in Figure 21, contains 
synchro units connected with the projector 
training mechanism and with the ship's gyro 
compass. They drive dials to indicate both the 
ship's and the projector's true bearings. They 
are also arranged to furnish a signal which 


can be fed through a detector system to actuate 
the projector motor drive in such a way as to 
maintain the projector on whatever true bear- 
ing appears on the dial. This is analogous to 
the automatic tracking feature. In this case 
the ship's gyro keeps the projector on a pre- 
selected true bearing. 

The large knob at the left is used to synchro- 
nize with the ship’s master gyro compass when 
the equipment is first set up. From then on, 
the left-hand dial reads the same as the ship's 
compass and gives the true course of the ship. 
The right-hand dial shows the true bearing of 
the axis of the projector at all target-following 
training speeds. The lower right-hand control 
switches the circuit from automatic target 
tracking to maintenance of true bearing. When 
switched to the latter, the projector automati- 
cally maintains itself at whatever true bearing 



Figure 21. Maintenance-of-true-bearing [MTB] 
cabinet used with listening equipment. 

appears on the right-hand dial. In this condi- 
tion the dial may be turned to change the true 
bearing if desired. 

3 performance and conclusions 

The 692 submarine sonar, as an experimen- 
tal system, has been found of value in deter- 
mining the feasibility of features considered as 
requirements for the ultimate submarine sonar. 
In general, it can be considered that the 692 
sonar has come through the development stage 
and is now ready for a Navy experimental pro- 
gram which will properly evaluate its capabili- 
ties. Some of the features, particularly those 
involving projector training and listening, have 


PERFORMANCE AND CONCLUSIONS 


139 


been studied by engineers in the field. How- 
ever, the short pulse echo-ranging and mine- 
detection equipment has not as yet been put 
aboard a ship. 

The listening system covers a range from 
200 c to 60 kc, and suitable listening bands can 
be selected on a continuous basis. The listen- 
ing ranges are limited under most operating 
conditions only by the ratio of signal to ambi- 
ent noise at the projector. Wide frequency 
range and low internal noise are also features 
of the WFA submarine sonar, which uses a 
somewhat similar projector. 

Calculations indicate that an interfering 
sound source of equal intensity has no appre- 
ciable effect on the target bearing obtained 
by 692 sonar until the interference is within 
zb 12 degrees at the lower frequencies and zb 5 
degrees at the higher frequencies. Limited tests 
indicate that such resolution can be obtained in 
practice. 

The projector training system has been 
found to be quite satisfactory from a functional 
standpoint; its low self-noise level does not 
interfere with listening and is below ambient 
noise under most conditions. Combined with 
the use of a BDI, it permits reading target 
bearings to within 0.1 degree. The hand con- 
trol is of the slewing type in that it varies the 
speed of train rather than having the projector 
follow a setting on the control. It was found 
quite satisfactory under the usual operating 
conditions. In this respect it is like steering a 
car where a given turn of the wheel produces 
a constant rate of change of direction, not a 
new direction, as would be obtained with a 
follow-up type of control. 


Trials of the automatic target tracking fea- 
ture under ideal conditions at a lake showed 
an average bearing lag error of 0.03 degree 
with a standard deviation of 0.08 degree. At 
sea, where such variables as water transmission 
conditions and actual sound target location are 
involved, the standard deviation of the sound 
bearings with respect to the stern of the target 
was about 0.15 degree. This means that for 
practical purposes sound bearings obtained by 
the 692 sonar are well within zb 0.5 degree of 
the actual relative bearing of the sound source. 
The test data contain errors inherent in the 
measuring means, so that the absolute accuracy 
of the system is probably better than indicated. 
The accuracy of the 692 sonar training system 
was such that similar training systems were 
used in a trial model of a triangulation ranging 
equipment. 

The continuous search feature was found 
helpful for locating targets before switching 
to automatic target tracking. Experience indi- 
cated that this feature should also be of con- 
siderable aid during evasive tactics, as it dis- 
plays the true and relative bearings of all at- 
tacking ships within sound range. The continu- 
ous search feature is also suitable for torpedo 
detection and tracking. However, considerably 
more data are required before its worth in this 
connection can be evaluated. 

Although the listening system does not pro- 
vide a direct observation of target range, it 
was found that by using the sound-level meter 
indication and by determining the frequency 
shift required to give a fixed loss in level, the 
target range could be estimated with reason- 
able accuracy. 




Chapter 12 


SUBMARINE TRIANGULATION-LISTENING-RANGING [TLR] SYSTEM 



Figure 1. Components of the TLR system. 


The submarine triangulation-listening-rang- 
ing [TLR] system, developed by CUDWR-NLL, 
is used to detect and determine the range and 
bearing of surface ships from a submerged sub- 
marine and indicate range on a recorder. The 
system includes two directional hydrophones 
mounted about 2W feet apart, right-left indi- 
cator [RLl] and listening equipment, automatic 
target follower [ATF] mechanisms, a triangle 
solver, and a chemical range recorder. The 
hydrophones are 5 feet long, straight, electri- 
cally split, magnetostriction units without lobe 
reduction. The RLI and listening equipments 
use the sum and difference signals from the 
two halves of each hydrophone to provide sonic 
listening, right-left indications, and actuation 
of the ATF mechanisms. The triangle solver 
makes use of the difference in target bearing 
from the tivo hydrophone stations to determine 
the target range. 


bearing and range information ordinarily ob- 
tained by means of periscope or radar observa- 
tions, with a final range check sometimes 
obtained by means of a single ping from the 
echo-ranging gear. An anticipated increase in 
the scope of enemy antisubmarine operations 
was expected, however, to increase greatly the 
danger of single-ping echo ranging and of radar 
mast or periscope exposure at ranges less than 
3,000 yards. 

Although accurate bearings can be obtained 
by sonic or supersonic listening methods, no 
means were available to obtain range informa- 
tion without the use of periscope, radar, or 
echo ranging observations. The necessity for 
discontinuing use of these methods of observa- 
tion within 3,000 yards of the target would 
deprive the submarine's attack team of range 
information during the vital closing phase of 
an attack. 

Preliminary investigations indicated that it 
might be possible to base such a ranging system 
on triangulation by listening, and a program 
was consequently undertaken. 

This program led to the development of a 
triangulation-listening -ranging [TLR] system. 
It consists essentially of two listening stations 
a known distance apart aboard a submarine. 
Accurate target bearings from these stations 
can be translated into range by triangulation. 
The system is capable of determining ranges 
silently within about zt 10 per cent out to 3,000 
yards for targets within ± 50 degrees of the 
beam. This accuracy is believed to be better 
than that usually obtainable by means of the 
periscope and is considerably better than the 
original seemingly difficult objective of d= 10 
per cent accuracy at 1,000 yards range. 


121 INTRODUCTION 

I N ORDER TO EXECUTE a successful torpedo 
attack, it is necessary for a submarine to 
have accurate information concerning the 
enemy vessel's course and speed. Determina- 
tion of these two factors involves the use of 


122 ANALYSIS OF THE PROBLEM 

Prior to the start of development work, 
studies were made of the tactical requirements 
governing or limiting the use of triangulation 
systems and of the bearing accuracies required. 



DEVELOPMENT 


141 


A triangulation system of the type under con- 
sideration is. inherently most accurate when the 
target is on the beam of the submarine. As the 
target bearing approaches the bow or the stern, 
range accuracy decreases until the system be- 
comes ineffective. A current study of sub- 
marine tactics revealed that the submariner is 
instructed to attempt to keep the target abeam 
as long as possible. Only near the end of an 
attack run just before firing or in certain spe- 
cial circumstances does the target bear close 
either to bow or to stern. 

The range errors corresponding to bearing 
errors of various magnitudes for targets at 
several different relative bearings and ranges 
were determined both by calculations and by 
graphical means. From these determinations it 
was concluded^ that, at a range of 1,000 yards, 
triangulation ranging might be anticipated to 
have a probable error of 12 per cent or less 
under the following conditions: (1) Standard 
deviation of bearing error must not exceed 
zb 0.25 degrees. (2) The system must be ca- 
pable of taking bearing data rapidly enough to 
provide the number of readings necessary to 
obtain a reasonable average over a period of 5 
to 10 seconds. (3) The relative bearing of the 
target from the submarine must lie within the 
limits 040 to 140 degrees or 220 to 320 degrees. 

If accuracies better than zb 0.25 degree can 
be realized, the range accuracy increases ac- 
cordingly and the usable arc is increased. 
When the target is directly on the beam, a 
range accuracy of 8 per cent may be antici- 
pated at 1,000 yards for a bearing error (stand- 
ard deviation) of zb 0.25 degree. 

A discussion of these limitations with sub- 
marine commanders having recent patrol ex- 
perience indicated that, while a usable arc of 
zb 50 degrees from either beam was considered 
acceptable, it was believed desirable, if possible, 
to reduce the anticipated bearing errors suffi- 
ciently to attain maximum probable range 
errors not greater than about 12 per cent at 
2,000 yards (instead of at 1,000 yards) in 
order to provide range accuracies equal to or 
better than those obtained with the periscope. 

In order to obtain bearings of the accuracy 
required for triangulation ranging, it was 
recognized ^s necessary to incorporate in the 
hydrophone system a means of providing for 


bearing deviation indication. Several BDI sys- 
tems'‘ had previously been developed for use in 
connection with other types of listening or 
echo ranging equipment and a comparison was 
made of four such systems, each of which 
makes use of the difference in time of arrival 
(hence phase) of the incident sound at two 
hydrophone elements k small distance apart. 
On the basis of this study a BDI system desig- 
nated as the right-left indicator [RLI], as de- 
scribed in Chapter 10, was selected as requiring 
the least amount of additional design and con- 
struction time and as having a possible inher- 
ent advantage in that the basic vector relation- 
ships are established in the input circuits. 

DEVELOPMENT 

Preliminary Surface Ship Tests 

The fundamental elements of an experi- 
mental TLR system were first installed on a 
127-foot long surface patrol ship. In this instal- 
lation two hydrophones, consisting of straight, 
two-section magnetostriction units, were lo- 
cated 120 feet apart on shafts mounted overside 
from platforms constructed as far forward and 
aft as feasible. A power training system with 
an automatic target follower fATF], and bear- 
ing repeater facilities was provided for each 
hydrophone. The RLI equipment, indicated in 
the block diagram. Figure 2, provided for opera- 
tion in the band 5- to 9-kc and for wide-band 
listening from either channel. 

The initial tests made with this equipment 
were directed toward determining the bearing 
accuracy obtainable with hydrophones from 3 
to 6 feet in length. To facilitate comparison 
between bearings obtained sonically and the 
actual (or optical) bearing of a target vessel, 
an instrument known as a direct deviation indi- 
cator [DDI] was developed. The DDI consists 
essentially of a telescope mounted with the eye- 
piece directly above the hydrophone shaft cen- 
ter line and with its axis accurately lined up 
with the center line of the major lobe of the 


“ Discussed also in Division 6, Volume 15. 

^The systems compared are designated as simulta- 
neous lobe comparison [SLC], phase-actuated locator 
[PAL], right-left indicator [RLI], and the NRL sys- 
tem originated by the Naval Research Laboratory. 2 


142 


SUBMARINE TRIANGULATION-LISTENING-RANGING [TLR] SYSTEM 


hydrophone. The telescope covers a field of 4 
degrees and is provided with a reticle having 
0.1-degree divisions. The eyepiece of the tele- 
scope can be fitted with a 35-mm or 16-mm 
camera for photographing the target vessel to 
determine the deviation between optical and 
sonic bearings. 


readings showed an accuracy of ± 0.21-degree 
standard deviation, while groups of 20 read- 
ings gave standard deviations of ± 0.05 degree 
to ± 0.14 degree. These data were obtained 
while tracking a tugboat presenting a beam 
aspect at 3,000 yards on a fiat sea with no 
interfering targets. The results are summarized 


ELECTRICALLY SPLIT 



Most of the surface ship bearing-accuracy 
tests were made with the vessel at anchor or 
drifting in a relatively calm sea. Various small 
craft served as targets, circling the listening 
ship at speeds of 6 to 10 knots and ranges of 
300 to 2,000 yards. Occasional observations 
were also made on freighters or other traffic 
passing the operating area at ranges up to 
5,000 yards. 

Preliminary tests were made using a 6-foot, 
split JP-type hydrophone described in Chapter 
10, an experimental RLI, and listening ampli- 
fier, and hand training at a 1-to-l ratio. An 
analysis of four groups of observations, con- 
taining 35 to 352 readings, showed standard 
deviations of d= 0.47 degree with maximum 
deviations of 1.3 degrees from the correct 
bearing. Subsequent tests using 4-foot straight 
hydrophones and hand training through a 20- 
to-1 reduction gear gave a standard deviation 
of d= 0.55 degree for a group of 69 readings; 
and smaller groups of 15 successive readings 
resulted in standard deviations up to zb 0.62 
degree. 

After application of an ATF mechanism, 250 


in the histogram. Figure 3. Bearing accuracies 
were determined with the DDI by analyzing 
exposures at half-second intervals. Numerous 
other motion pictures taken on calm days 



DEGREES 

Figure 3. Histographic summary of surface ship 
bearing accuracy test. 

showed that the bearing accuracies for 10- 
second periods, picked at random from a 5- 
minute period, were of the order of zb 0.15- 




o 


DEVELOPMENT 


143 


degree standard deviation when no interfer- 
ence was present. 

Analysis of the results of these tests made it 
possible to define many of the factors affecting 
tne accuracy of sound bearings measured with 
a system of this type and to estimate their 
probable effect upon ranges obtained by tri- 
angulation. Among these factors, the follow- 
ing appear to be the more important: (1) own 
ship’s noise; (2) random water noises, direc- 
tional distribution of which may not average 
out over short periods of time; (3) wake effects 
which may obscure the noise from the target’s 
screws; (4) horizontal refraction due to cur- 
rents of differing densities; (5) interfering 
targets; (6) the lag between the actual bear- 
ing of a moving target and the sound bearing; 
and (7) system errors. 

The influence of some of these factors, such 
as wake effects and own ship’s noise, were ex- 
pected to be reduced when operating from a 
submerged submarine. The bearing errors 
caused by horizontal refraction and lag due to 
speed of the target are likely to be small and, 
since both hydrophones of a TLR system are 
equally affected, these effects would not be ex- 
pected to introduce significant range errors. 
Interfering targets may cause bearing errors 
of considerable magnitude under certain condi- 
tions, particularly when the interfering signal 
is of comparable or greater intensity than that 
of the intended target and when the bearings of 
the two signals are close together. 

The system errors which appear to be of 
most importance are: (1) lack of adequate 
matching in frequency sensitivity and phase 
characteristics of the two hydrophones and sum 
and difference networks, (2) mechanical back- 
lash coupled with torsional and fiexural strains 
in the hydrophone shafts which may cause thh 
system to hunt, (3) bearing repeater errors 
due to inherent lag in the servo system, (4) 
overloading, from lack of adequate automatic 
gain control, which can cause false RLI and 
ATF deflections, and (5) the electric time con- 
stants in the RLI circuits which may cause 
hunting unless they are carefully chosen. 

These preliminary surface ship tests indi- 
cated that, with a system having power train- 
ing, RLI, ATF, and 4-foot split hydrophones, 
the original objective of bearing accuracies 


within =b 025-degree standard deviation could 
be realized for single targets and could prob- 
ably be improved to about dz 0.15 degree by 
careful control of the system errors. 

Preliminary Submarine Tests 

It was apparent that similar tests should be 
conducted on a submarine to evaluate the effect 
of screw noise and hull vibration and to deter- 
mine whether special methods would be re- 
quired to permit ATF operation at speeds up to 
3 knots. 

A topside through-the-hull training gear was 
consequently installed for this purpose at a 
point a few feet forward of the screws on the 
submarine USS S48. In order to determine 
how much noise was transmitted to the hydro- 
phone mechanically through the hull and how 
much was waterborne, a double hydrophone 
mount was used to allow the mounting of one 
hydrophone rigidly on the training shaft and 
another just above it on a vibration-isolating 
mount. Two 3-foot long split hydrophones 
mounted in JP-type baffles were used with an 
approximate 15-foot distance between the 
screws and the hydrophones. 



I 

TUG AND TOW AT AVERAGE RANGE OF 
1500 YARDS 

NUMBER OF SOUND OBSERVATIONS N>I50 



MEAN DIFFERENCE BETWEEN SOUND 

AND PERISCOPE BEARING (UNCORRECTED 
FOR PARALLAX )-0.8* 

L. CTAMHARn nPUIATinw rt IKO 





SOUND READINGS EVERY 
3 SECONDS 






^ — s 

lOUND E 

BEARING 

IS 

PERISCOPE BEARINGS- 
















I7«30 18:30 19:30 20<30 2I«30 

TIME 

Figure 4. Comparison of sonic and periscope bear- 
ings for a typical run. 

Initial listening tests, using RLI and power 
training equipment, indicated that at speeds of 
3 knots and below screw noise was not serious 
with either the rigidly mounted hydrophone or 
the isolated unit. At speeds above 3 knots, cavi- 
tation was present and the noise level was high 
with both units. At all speeds, operation of the 




144 


SUBMARINE TRIANGULATION-LISTENING-RANGING [TLR] SYSTEM 


stern planes caused noise that could be vir- 
tually eliminated by hand operation of the 
planes. The water noise at 3 knots and peri- 
scope depth was found to be low. Targets could 
usually be followed without difficulty to ranges 
of 4,000 to 7,000 yards from the submarine. 

Bearing accuracy tests with this system 
showed average errors of d= 0.25-degree stand- 
ard deviation. On one run, in which sound 
bearings were taken every 3 seconds on a tug 
at an average range of 1,500 yards, the stand- 
ard deviation of bearing error, after correc- 
tions for parallax, was ± 0.15 degree. The re- 
sults of this test are shown graphically in 
Figure 4. 

12 4 EXPERIMENTAL SYSTEMS 

USS SU Installation 

A complete TLR system, incorporating a tri- 
angle solver mechanism and a range recorder, 
was built and installed for further testing on 
the USS SA8. This system provided (1) mini- 
mum mechanical backlash in the hydrophone 
drives, (2) a bearing-repeat servo system ac- 
curate within about =b 0.03 degree, (3) an A VC 
system capable of stabilizing adequately the 
sound system in the presence of pulsating sig- 
nals such as are usually generated by the 
screws of large ships, (4) an ATF, (5) con- 
tinuous recording of ranges up to 4,000 yards 
on a moving chart and, (6) remote operation 
of the hydrophone system to allow control from 
one main station. 

The design and construction of the triangle 
solver with its associated servo systems, the 
hydrophone drives, and the range recorder 
were assumed by the Sperry Gyroscope Com- 
pany. 

The equipment manufactured by Sperry for 
the S^8 installation ^ consists of dual Ward- 
Leonard hydrophone servo and drive systems, 
dual hydrophone bearing-repeat systems, and 
an associated triangle solver and range re- 
corder, as indicated in the block diagram in 
Figure 5. 

In this installation, the original aft training 
gear, having proved satisfactory in the earlier 
tests, was retained and a forward hydrophone 
installation was made between frames 22 and 


23 to provide a base line of 205 feet between 
stations. The main control station was located 
in the motor room. A block schematic of the 
TLR system installed on this submarine, show- 
ing the forward hydrophone station and the 
components common to both stations, is pre- 
sented in Figure 5. 

Hydrophone Drives and Servo- Amplifiers, 
The servo system for each hydrophone incor- 
porates an amplifier which feeds the field of a 
d-c generator whose output controls a d-c motor 
geared to the hydrophone shaft. Three methods 
of control are provided, search, hand, and ATF. 
In the search position, a potentiometer pro- 
vides zero signal when in mid-position and pro- 
vides smooth variation of the signal to give 
maximum training speed to right or left at the 
limit stops on the control. In the hand position, 
when the hydrophone bearing-repeat dials are 
moved by hand cranks from the actual hydro- 
phone bearing, the error signal set up in the 
connected follower-synchro causes the hydro- 
phone to rotate to the new bearing. In the ATF 
position, the output of the RLI sound stack is 
directly connected to the hydrophone servo-am- 
plifier and causes the hydrophone to track the 
sound signal. 

The Ward-Leonard system of hydrophone 
training control is well adapted to this applica- 
tion. The inclusion of a small permanent-mag- 
net generator geared to the hydrophone shaft 
provides reverse feedback to the servo-amplifier, 
which allows large torque to take care of pos- 
sible binding in the through-the-hull shaft and 
at the same time prevents excessive overrun- 
ning of the shaft and allows precise control of 
hydrophone training speed. The control by this 
system is extremely smooth and the sensitivity 
is excellent. 

The bearing-repeat system consists of 1-to-l 
and 36-to-l synchro-generators connected to the 
hydrophone shaft, with similar control syn- 
chros on the bearing-repeat shafts geared di- 
rectly to the triangle solver. A triangle solver 
servo-amplifier associated with each hydrophone 
bearing-repeat system provides repeated bear- 
ings accurate to a fraction of a tenth of a 
degree. Inverse-feedback generators and a 
smoothing circuit incorporated in the amplifier 
permit high sensitivity, freedom from hunting 
and sharp resonances, and considerable smooth- 


EXPERIMENTAL SYSTEMS 


145 


ing of the fluctuations of the hydrophone when 
ATF is being employed. Bearings of the hydro- 
phone shaft are repeated in this manner when 
ATF and search are used. For hand-controlled 
training, the dial bearings are repeated to the 
hydrophone shaft directly by the hydrophone 


used for computing range at fixed coast-defense 
gun emplacements and is modified to make it 
adaptable to this application. The solution of 
the equation 


Range = 


sine after an gle 
sine difference angle 


X baseline 



servo-amplifier as described above, and the tri- 
angle solver servo-amplifier is not used. 

Triangle Solver and Recorder. The triangle 
solver is a mechanical device which has been 


gives the range from the forward hydrophone 
to the target. This is accomplished mechanically 
by means of sine and multiplier cams suitably 
geared together so that when the value of range 






146 


SUBMARINE TRIANGULATION-LISTENING-RANGING [TLR] SYSTEM 


is not such as to. satisfy the above equation, a 
contact is closed which varies the value of 
range until it is satisfied. The values of range 
so inserted are transmitted by a fiexible shaft 
to a range recorder. A continuous trace of 
range up to 4,000 yards is recorded on spe- 
cially ruled paper moving at a rate of approxi- 
mately % inch per minute. 

Hydrophones and RLI System. The re- 
mainder of the equipment includes the 3-foot 
straight split hydrophones, JP-type hydro- 
phone baffles, a preamplifier at each hydro- 
phone station, dual RLI units operating in the 
5- to 9-kc band, and an a-c power supply and 
audio unit at the main control station. The 
sound system functions identically for either 
hydrophone station. 

After taking the quadrature sum and differ- 
ence signals in the preamplifier input trans- 
formers, each channel is amplified 60 db. This 
value of fixed gain avoids the complexities of 
remote control of gain from the main station 
and is considered a good compromise to mini- 
mize overloading on extremely loud signals and 
yet obtain sufficient amplification of extremely 
weak signals to override local electric noise 
pickup. The minimum usable signal is assumed 
to be 0.25 fxv per hydrophone half and this 
becomes 250 /xv at the output of the preampli- 
fiers. The cathode-follower outputs of the pre- 
amplifiers are fed to the main control station 
where they pass through 0- to 20-db step pads 
before entering the RLI unit. 

In the RLI unit, 0.5- to 12-kc band-pass filters 
are provided to remove low-order 60-c har- 
monics which would be troublesome in the 
audio-listening channel and to help prevent 
blocking by surface ship echo ranging. The 
filters are followed by a varistor gain control 
circuit which is controllable over a 38-db range 
either manually or by A VC. As it was desired 
that the gains of the two channels always track 
within 2 db, varistors were chosen for con- 
trolling automatically the gain of the sum and 
difference channels. Following the varistor 
gain control, the signals are amplified in sepa- 
rate two-stage amplifiers. At this point, either 
channel of either station may be selected by a 
switch and amplified to provide audio listening. 
The signals are also fed to phase-shifting net- 
works which, at the mean frequency, shift the 


phase of the sum channel 45 degrees and the 
difference channel 135 degrees to provide a 
relative phase shift between channels of 90 
degrees. 

This network resolves the quadrature com- 
ponents (obtained after taking sum and differ- 
ence) so that the sum and difference signals 
become in-phase or 180 degrees out-of-phase, 
depending upon whether the hydrophone is 
trained to the left or right of the target. The 
use of a phase-shifting network which accu- 
rately shifts the relative phase of the two chan- 
nels by 90 degrees is desirable for two reasons. 
Unbalance of the two halves of the hydrophone 
causes errors in right-left indications if the 
phase shift varies considerably from 90 degrees ; 
and a phase shift varying considerably with 
frequency may cause error if the slope of the 
target noise spectrum is appreciably different 
from the slope of the noise source used for sys- 
tem lineup. This latter consideration is re- 
garded as important, because the slope of the 
target noise spectrum may change with range 
as well as from target to target. 

The signals, now 0 or 180 degrees apart, are 
passed through a 5- to 9-kc filter and a two- 
stage amplifier and combined in the first phase 
detector to give d-c output at low level. Be- 
cause of the difficulty of maintaining balance in 
a straight d-c amplifier, the d-c output of the 
phase detector is converted to 60-cycle alter- 
nating current in a Brown converter and then 
amplified. This signal is again rectified in the 
second phase detector to provide the d-c output 
necessary to give right-left indications on a d-c 
zero-center microammeter (full-scale deflection 
for 1 degree off target) . The 60-cycle a-c sig- 
nal is taken off ahead of the second phase de- 
tector to feed the Sperry hydrophone servo- 
amplifier for ATF operation. 

Preliminary Tests. The first tests of this 
system were made with the SU8 lying on the 
bottom with the hydrophones submerged to a 
depth of about 20 feet. During this period 
small hydrophones, located on the conning 
tower, were used as noise projectors to check 
the lineup of each hydrophone station. It was 
found that hydrophone bearings obtained on 
these noise sources were unreliable, and it was 
concluded that this was due to hull and conning 
towbr reflections and perhaps also to surface 


EXPERIMENTAL SYSTEMS 


147 


reflections. This condition was later corrected 
by locating the projectors within 15 feet of 
each hydrophone. In order to smooth out the 
trace on the range recorder, the range rate of 
the triangle solver was reduced from 150 knots 
to 60 knots by reducing the speed of the solver 
motor. 

In subsequent operations with live targets, 
ranges determined by periscope and by echo 
ranging from the target vessel were compared 
with ranges obtained on the TLR system. These 
tests indicated the need for: (1) a range stand- 
ard which would be accurate within about ± 3 
per cent out to 2,000 to 4,000 yards, (2) 
further smoothing of the triangulation system 
to give more uniform range traces and, (3) 
more discrimination against the effects of in- 
terfering targets. It was found that, in areas 
free of interfering targets, ranges up to 3,500 
yards on the beam could be obtained under con- 
ditions of optimum adjustment of all system 
components. Figure 6 shows a tracing of one 
of the best range-time records made during 
this period and illustrates the rapid fluctuations 
of indicated ranges. 

Revisions to the System. The brass shaft of 
the forward training gear was replaced by a 
stainless-steel unit providing twice the tor- 
sional stiffness, and the brass roller bearings 
were replaced by a steel ball race which halved 
the torque requirements to 12 pound-feet. Four- 
foot hydrophones, having full lobe reduction 
and equipped with JP-type baffles, were in- 
stalled in place of the 3-foot units. The fre- 
quency band of the RLI portion of the system 
was changed from 5 to 9 kc to 9 to 16 kc. 

The hydrophone drive response was decreased 
from 30 degrees per second to 4 degrees per 
second, the response of the range solver was 
further reduced to a maximum range rate of 
35 knots, the recorder paper speed was in- 
creased 20 per cent, and the balancing circuit 
of the solver servo-amplifiers was improved. A 
modification to the electronic listening and 
servo systems was evolved so that, during ATF 
operation, a scanning action of the hydrophones 
was obtained. In order to improve the accuracy 
of reference ranges with which to compare 
values obtained with the TLR, a more efficient 
and seaworthy radar reflector and towable buoy 
assembly was constructed and the target ship 


was equipped with a type SU radar as the pri- 
mary range reference beyond 2,500 yards. 

Because most of these changes were intro- 
duced gradually during intermittent periods 
of availability of the submarine, the effect of 
each modification was observed separately. 
Each change was found to contribute to the 



Figure 6. Tracing of an early range-time record 
illustrating the rapid fluctuations of range indica- 
tion. 


accuracy and reliability of the system. In the 
case of the scanning action of the hydrophones, 
the greatest benefit was realized when the fre- 
quency and amplitude were confined to 4 c and 
1/2 degree, respectively. 

Appraisal Tests and Demonstrations. For 
final tests and appraisal of the system, a 4-week 
availability period of the USS SA8 was sched- 
uled. In the last 3 weeks of this period, a sec- 


148 


SUBMARINE TRIANGULATION-LISTENING-RANGING [TLR] SYSTEM 


ond surface ship was assigned to work with 
the target vessel ordinarily used so that the 
performance on single and multiple targets 
could be evaluated. 

With the submarine generally running at 80- 
foot keel depth and 3 knots, the ranges on a 
single target, at 3,000 yards and within d= 50 
degrees from either beam, were accurate within 
15 per cent about three-fourths of the time. 
Figure 7 shows one of the best range record- 



Figure 7. One of the best range recordings ob- 
tained with the SA8 TLR system. 


ings made on a single 15-knot target. Refer- 
ence data obtained by radar and echo ranging 
from the target ship are included for compari- 
son. The deviation between TLR and radar or 
echo ranges does not exceed 10 per cent out to 
3,500 yards. 

Comments of observers varied from favor- 
able to enthusiastic, with the following points 
representative of the conclusions. (1) The de- 
velopment of the TLR system should be con- 
tinued with emphasis on greater reliability, 
provision of calibration facilities, and better 


discrimination between multiple targets. (2) 
The necessary target resolution was not pre- 
cisely defined, but submarine commanders hav- 
ing war-patrol experience commented infor- 
mally that the ability to discriminate between 
equal-strength targets separated 10 degrees or 
more would probably be satisfactory. (3) A 
tactical evaluation, preferably under patrol 
conditions, was considered essential. 

Studies of Hydrophones 
AND Interfering Targets 

While tests and demonstrations of the S^8 
TLR system were in progress, separate devel- 
opment work led to the design of a 5-foot long 
split hydrophone designated as the NL-124 
type, described in Division 6, Volume 11. This 
unit consists of 10 toroidally wound permanent- 
magnet magnetostriction elements mounted 
end-to-end on a stiff brass rod. The sensitivity 
averages 17 db higher than that of the 3-foot 
long JP-type hydrophone. When the sensitivi- 
ties of the five elements comprising each half 
are selected and averaged, the amplitude bal- 
ance between halves can be held to within 1 db. 
Varying degrees of lobe reduction may be in- 
corporated by decreasing successively the sen- 
sitivity of the elements toward the ends of the 
unit. 

An extended series of tests was conducted on 
the surface ship experimental TLR installation 
to determine the effects on TLR ranges of inter- 
fering targets at various intensity levels and 
angular displacements relative to the desired 
target. An artificial equalized noise source was 
located in quiet water about 60 feet deep and 
about 150 yards from the experimental TLR 
equipment, so that RLI patterns of 3-, 4-, and 
5-foot split hydrophones having full, inter- 
mediate, and no lobe reduction were deter- 
mined. 

The initial tests were made using the 5- to 9- 
kc frequency band. A graphical method was 
developed whereby one RLI pattern could be 
superimposed on another but displaced by vary- 
ing amounts to simulate angular and amplitude 
differences comparable to those which might be 
expected from ships in a convoy. The results 
obtained by this method checked quite closely 
with measured interference effects using two 


EXPERIMENTAL SYSTEMS 


149 


artificial signals. It was found that appreciable 
bearing errors were introduced, particularly 
for high interfering levels, when the targets 
were separated by angles less than the angle 
between the on-target and secondary zeros of 
the RLI pattern. These values, as measured 



Figure 8. Sum, difference, and RLI patterns of 
NL-124 hydrophone without lobe reduction for 9- 
to 16-kc band. 


using 5-foot hydrophones and the 5- to 9-kc 
band, were about 17 degrees for full lobe re- 
duction and 13 degrees for no lobe reduction. 

The NL-124 hydrophone without lobe reduc- 
tion was also tested with an RLI modified to 
operate in the 9- to 16-kc band. Figure 8 shows 
the sum and difference directivity character- 
istics of this hydrophone unit and the resulting 
RLI characteristic. In this case, the separation 
between the on-target and secondary zeros of 
the RLI response is only 6.5 degrees. Figure 9 
shows the probable error introduced by inter- 
fering targets of various signal strengths and 
angular positions with respect to the main tar- 
get. When using this unit, strong interference 
less than 6 degrees from the target can still 
introduce an appreciable bearing error. Greater 
separations do not appear to be serious. 

Fleet Submarine Installation 

As a consequence of the encouraging results 
obtained in the tests of the S48 TLR system, 
the Bureau of Ships requested that this equip- 
ment be modified to permit its installation on 
a new-construction submarine for evaluation of 
its performance under patrol conditions. Be- 
cause of the limited time available for adjust- 


ing and testing the system after installation, it 
was decided that no basic design changes other 
than in the training assembly should be made. 
Every effort was made, however, to improve 
the performance of those elements of the sys- 
tem which previous tests had shown to be de- 
ficient and to anticipate the operational and 
maintenance problems which might arise dur- 
ing war patrol. ' 

Since the unit was still considered to be pri- 
marily an experimental model, it was believed 
necessary to have all the operational parts 
readily accessible. For this reason the main 
control station is located in the maneuvering 
room where access to three sides of the main 
stacks is possible. 

The forward hydrophone station is located 
between frames 22 and 23 and the after station 
between frames 123 and 124. This arrangement 
results in a base line of 233.5 feet, an increase 
of 14 per cent over that of the S48 installation. 
The training assemblies are equipped with 
extra-heavy stainless-steel shafting in order to 
attain a high degree of torsional rigidity. The 



Figure 9. Probable angular errors due to inter- 
fering targets when using NL-124 hydrophone 
without lobe reduction in the 9- to 16-kc band. 


drive motor, repeater synchro, and spur gear 
assembly is bolted to a support which is ma- 
chined so that the drive pinion is automatically 
aligned and meshed tightly with the main 
driven gear on the shaft, thereby eliminating 
backlash at this critical point. The motor and 
gearbox frame is bolted rather than welded to 
a flange which terminates the casing of the 



4 


150 


SUBMARINE TRIANGULATION-LISTENING-RANGING [TLR] SYSTEM 


training assembly in order to eliminate weld- 
ing strains during installation and simplify 
field installations.^ 

The main operating station in the maneuver- 
ing room is functionally centralized, containing 
all components requiring adjustment during 
operation. Transmission of range and bearing 
information to the conning tower is by means 
of a range recorder and a synchro system which 
repeats bearings from the forward and after 
stations. The conning tower components are 
located so that TLR data are available for the 
conning officer, plotting officer, and the torpedo 
data computer [TDC] operator during attack 
or evasive maneuvers. An intercommunicating 
system provides for exchange of operational 
data between the conning tower and the TLR 
operator. 

RLI and Listening Equipment. The RLI and 
listening equipment provided for this installa- 
tion is basically the same as that finally incor- 
porated in the S-^8 system and indicated in the 
block diagram. Figure 5, but a number of modi- 
fications are incorporated to improve the per- 
formance and adapt the equipment to the re- 
quirements of a new-construction boat. 

The hydrophones supplied are of the 5-foot 
NL-124 type with no lobe reduction. These 
units, having maximum sensitivity in the 9- to 
16-kc band, are equipped with NL-129A type 
baffies, described in connection with submarine 
listening equipment, which provide a front-to- 
back differential of 20 db to 23 db in that 
region. The aft hydrophone and baffie assembly 
is shown in Figure 10. 

New preamplifiers were constructed, having 
reduced electric noise pickup and transformer- 
coupled output. 

Equalizers are used which provide a positive 
slope of about 6 db per octave to the overall 
frequency characteristic (hydrophone and am- 
plifiers) up to 16 kc, to compensate for the 
negative slope of the average screw noise spec- 
trum. This leads to the summation of a wider 
band of frequencies, and the resulting sum, 
difference, and RLI response patterns should 
be narrower than those shown in Figure 8, 
with consequent reduction of interference from 
secondary targets. The listening channel is pro- 
vided with high-pass filters having cutoffs at 


500, 1,500, and 4,500 c, which appear to be 
optimum for use with 5-foot hydrophones. 

A new power-supply unit furnishes regulated 
power for the entire sound amplifier system 
and incorporates a signal generator which sup- 
plies noise of proper frequency distribution to 
the test projectors, located topside near the 
hydrophones. The 150- and 275-volt outputs are 
constant to within 0.5 volt for a-c line varia- 
tions from 90 to 130 volts, as a precaution 



Figure 10. Aft TLR hydrophone installation on a 
fleet-type submarine. 


against the introduction of transients in the 
RLI output. The sonar talkback system which 
provides communications between the main 
control station and the conning tower is a com- 
ponent of the Model JT sonar equipment. 

The main station sound equipment (for both 
hydrophones) is housed in a cabinet 43 inches 
high, 20 inches wide, and 11%2 inches deep, 
with the various chassis readily accessible by 


EXPERIMENTAL SYSTEMS 


151 


hinged or removable cover plates. A front view 
of this unit is shown in Figure 11. 



Figure 11. Main station sound equipment used on 
fleet-type submarine. 


mechanical adjustments (zt 1.0 degree) from 
the fine synchros to the exterior of the triangle- 
solver cabinet. 

One range recorder is installed with the main 
control equipment and a second recorder is 
installed in the conning tower. The main sta- 
tion control equipment is shown in Figure 12, 
and the range recorder in Figure 13. 



Figure 12. Main station control equipment used 
on fleet-type submarine. 


Servo-amplifier, Range Solver, and Recorder. Tests. Only preliminary adjustment and test- 
In the servo-amplifier, range solver, and re- ing of this TLR installation were possible prior 
corder equipment only a few relatively minor to the time responsibility for the development 
changes were made in adapting the components was transferred to the Naval Research Labo- 
used in the SJf8 installation for use on the new- ratory. Dockside measurements were made to 
construction submarine. Lineup of the bearing- determine the bearing accuracy capabilities of 
repeat dials to correspond to hydrophone bear- the system including the hydrophone drives 
ings was difficult in the SJt-8 installation and this and bearing-repeat synchros. The maximum 
was remedied by increasing the dead spot near bearing error of the system was thus deter- 
zero on the coarse synchros and bringing out mined to be 5.2 minutes, with a mean error of 


152 


SUBMARINE TRIANGULATION-LISTENING-RANGING [TLR] SYSTEM 


5.0 minutes. Tests of the electronic components 
indicated adequate performance in all respects. 

During a trip of the submarine to deep water, 
an opportunity was afforded to coordinate peri- 
scope and TLR ranges over a period of several 
hours. One run made at this time checked 


were made with the Submarine Signal Com- 
pany for the procurement of five preproduction 
units for additional war patrol appraisal tests 
on other new-construction boats. These units 
are designated by the Navy as Model XJAA 
listening-ranging equipment. 



Figure 13. Range recorder used on fleet-type submarine (cover removed). 


Drawings and specifications of the SJ^8 system 
and complete information on subsequent im- 
provements to this equipment were furnished 
to the contractor. Further assistance was also 
made available in the form of consulting serv- 
ices from engineers familiar with the original 
development work. 

None of the preproduction units was com- 
pleted prior to transfer of the project to NRL. 
The manufacturer’s plans at that time, how- 
ever, indicated that the XJAA units would in- 
corporate hydrophones, baffles, and calibration 
projectors identical with those used in the last 
laboratory installation. The designs prepared 
for RLI units also retain the basic features of 


within 6 per cent out to 4,500 yards. Owing to 
the limited data obtained and because no sys- 
tematic lineup of the TLR installation had pre- 
viously been possible, it is not believed that this 
run is necessarily typical of average perform- 
ance of the system. At the time responsibility 
for the system was transferred to the Naval 
Research Laboratory, a 14-day test period had 
been scheduled primarily for further opera- 
tional evaluation of the TLR system. 

12 5 PREPRODUCTION UNITS 

Concurrently with the work of transferring 
the experimental TLR system from the S^8 to 
a new-construction submarine, arrangements 


RECOMMENDATIONS FOR FUTURE DEVELOPMENT 


153 


the laboratory models, but the control station 
equipment layouts are designed to permit these 
components to be installed in the conning 
tower. 


*26 RECOMMENDATIONS FOR FUTURE 
DEVELOPMENT 

The experience obtained with the experi- 
mental units of this equipment led to the con- 
clusion that a number of improvements could 
be made by further development work. 

At 6 knots, the torque due to water drag on 
the hydrophone varies between 0 and 42 pound- 
feet, depending on the bearing, and at 3 knots 
the maximum torque is 10.5 pound-feet. It is 
of major importance that the system be capa- 
ble of operating free from the bias that can be 
caused by this torque and also, to some extent, 
by frictional forces. Tests indicate that the best 
means of minimizing these effects is to operate 
with overall system sensitivity below hunt yet 
sufficiently responsive so that a scanning action 
is obtained. A scanning frequency of about 4 c 
with an amplitude of zt 0.5 degree appears to 
approach a good averaging of the correct bear- 
ing. Further developmental work should in- 
clude a thorough investigation to determine the 
optimum dynamic response for reducing bias to 
the absolute minimum. 

Further evaluation of the experimental 
equipment on new-construction submarines may 
indicate the need for additional reduction of 
the effects of interfering targets. This is an 
inherent limitation to this type of system which 
niay be minimized, but cannot be entirely 
eliminated. It is believed that a clarification of 
the tactical requirements is needed in deter- 
mining the need for further work on this prob- 
lem. Action of the system in the presence of 
interfering targets is dependent on a number 
of factors: (1) the relative signal levels of the 
prime and interfering targets, (2) the angular 
separations of the targets, (3) the bearings of 


the targets relative to the submarine, (4) th^ 
design of the hydrophone and baffle assemblies, 
and (5) the frequency band in which the bear- 
ing deviation system operates. One method of 
minimizing the interference problem is by de- 
creasing the width of the main and side lobes 
of the hydrophone which may be accomplished 
by operating at a higHer frequency band than 
9 to 16 kc or by using a longer hydrophone. A 
system designed to operate at higher frequen- 
cies should be based upon determinations of the 
available target energy, the variations in the 
slope of the energy spectrum with range and 
type of source, and also on the feasibility of 
developing hydrophones of suitable sensitivity 
in the desired band. 

An ultimate limitation to the accuracy of a 
submarine triangulation system is the precision 
with which the hydrophones can be zeroed with 
reference to the base line. Alignment by means 
of a reference noise source mounted near each 
hydrophone is possible to within it 0.05 degree 
in the experimental system which means that 
an alignment error as great as it 0.1 degree 
may be present. For a base line of 233 feet, an 
error of this magnitude causes a range error 
of approximately 185 yards for a target at 090 
degrees relative bearing and 3,000 yards range, 
indicating the importance of increasing align- 
ment precision. In this connection, the advan- 
tages offered by installed ringed hydrophones 
should be investigated. 

In the experimental equipment, instabilities 
have been present which cause shifts in the zero 
alignment of the system. It is recommended 
that tests be conducted to determine whether 
this condition is due to system variations or is 
inherent in the medium. 

In addition to the possible system improve- 
ments noted above, it is believed that coordi- 
nation of the use of the triangulation system 
with submarine attack and evasive tactics may 
result in auxiliary uses, such as tracking sepa- 
rate vessels with each of the two hydrophone 
stations. 




Chapter 13 

TORPEDO DETECTION STUDIES AND SYSTEMS — MVP AND TDM 


rpoRPEDO NOISE generally occurs at an inten- 
X sity level high enough to be detected above 
ship’s self-noise and at a range great enough 
to warn the listening ship to maneuver in time 
to prevent the torpedo’s hitting its mark. The 
projects for merchant vessel protection [MVP] 
and WCA - 2 torpedo detection modification 
[TDM] provided information regarding tor- 
pedo noise and resulted in recommendations 
and equipment for torpedo detection. 

The MVP project consisted mainly of tests 
to ascertain what conditions existed. Much of 
the value of these tests and measurements lies 
in the fact that they included several types of 
gear and were not confined to special require- 
ments of sonic equipment. Analyses of noise 
from many types of torpedoes showed that 
the frequency distribution ranged from sonic 
to supersonic with maximum intensity of all 
types tested in the sonic range from 300 to 
800 c. Measurements of ship’s self-noise, com- 
pared with the torpedo noise measurements, 
showed that the optimum operating region is 
below 3 kc for a nondirectional pickup unit, 
but that detection is possible at higher fre- 
quencies for directional units. 

The conclusion that torpedo frequency com- 
ponents exist in the supersonic as well as the 
sonic region led to work on the TDM. With 
this knowledge, experimenters realized that the 
supersonic WCA-2 equipment, already installed 
on submarines, could be modified to detect tor- 
pedoes. This enabled quick installation of the 
new equipment without the addition of much 
gear to already crowded submarines. Another 
conclusion from the MVP tests recommended 
many features of the directional JP through- 
the-hull gear, patterned after a British con- 
tinuously rotating system studied. 

Sonic Protection for Merchant Vessels 
Against Torpedo Attacks [MFP] 

131 INTRODUCTION 

It was believed that if fast merchant ships 
could detect torpedoes at ranges of 1,000 yards 


or more and submarines at ranges up to 1,000 
yards, they could take action soon enough to 
reduce the possibility of being hit. This would 
preclude the necessity of being held down to 
convoy speeds of 10 to 12 knots. 

Two types of torpedo detection devices were 
already in use. The first of these,^ a commer- 
cial (Electro-Protective Corporation) fixed hy- 
drophone [E-P] system installed on a number 
of U. S. merchant ships, employed a hydro- 
phone on each side of the hull and gave an 
audible warning signal together with a visual 
indication of port or starboard approach. The 
second, a British device designed for warships, 
used a continuously rotated hydrophone in a 
streamline dome. This system provided both 
an audible indication and accurate bearing in- 
formation but required constant monitoring. 
Although a scanning equipment designed for 
screening anchored vessels against submarines 
(Anchored Vessel Screening, described in Divi- 
sion 6, Volume 15) was sufficiently developed 
to be considered for possible use on moving 
ships, this application of the equipment had 
not been investigated. Some information was 
available in British reports on the effectiveness 
of the rotating type of torpedo detection de- 
vice, but no reliable performance data existed 
for the E-P fixed hydrophone equipment. Fur- 
ther, no adequate information existed on two 
factors of fundamental importance to the tor- 
pedo detection problem: the self -noise (at or 
near the hull) of merchant ships and the noise 
from torpedoes. The program undertaken, 
therefore, consisted of (1) measuring the sound 
output of torpedoes, (2) measuring ship self- 
noise at various speeds, and (3) testing and 
evaluating each of the three types of equip- 
ment to determine its capabilities for detecting 
submarines from fast-moving merchant ships. 

Conclusions 

A number of broad conclusions were reached 
during this program. 

1. First it was concluded that anchored ves- 
sel screening equipment in its state of develop- 
ment at the time of testing was not adequate 




154 


TORPEDO NOISE MEASUREMENTS 


15S 


for detecting submarines at ranges up to 1,000 
yards from fast-moving merchant vessels. 

2. Sonic detection of submarine torpedoes at 
ranges up to about 2,000 yards from fast- 
moving merchant ships is feasible under mod- 
erate sea conditions with both the rotating and 
fixed hydrophone types of systems, but care 
must be exercised if ranges of this order are 
to be consistently obtained. 

3. Improvements to the fixed hydrophone 
system then available were necessary to in- 
crease its reliability for routine use on mer- 
chant ships. These improvements included rais- 
ing the recommended sensitivity settings, 
changing the circuit to improve selectivity be- 
tween torpedo and ship noise, and broadening 
the response characteristics of the hydro- 
phones. 

4. Compared with the fixed system, the 
rotating type had the advantages of more accu- 
rate bearing determination and greater pos- 
sible range but the disadvantages of greater 
complexity, increased difficulty of installation, 
and the necessity for constant monitoring. 


13.2 torpedo noise measurements 

A laboratory ship, used for the torpedo noise 
tests, was stationed approximately 2,000 yards 
from the torpedo firing point. The noise from 



Figure 1. Limits of noise from U.S. Mark XIV 
torpedoes. 


each torpedo was picked up by a 3A-type hydro- 
phone (Bell Telephone Laboratories) and re- 
corded from the instant of firing until the tor- 
pedo had passed 1,000 to 2,000 yards beyond 


the hydrophone. Analyses of the torpedo noise 
were made by playing back each recording 
through various filters, and plotting curves 



Figure 2. Limits of noise from U.S. Mark XVIII 
torpedoes. 



Figure 3. Limits of noise from British Mark VIII 
torpedoes. 



Figure 4. Noise from U.S. Mark XIII and Mark 
XIII-2A torpedoes. 




156 


TORPEDO DETECTION STUDIES AND SYSTEMS 


showing the pressure spectrum level within 
the range 200 c to 25,000 c for torpedo dis- 
tances of 1,000 yards and 1,500 yards on the 
approach runs. 

Examination of the noise spectra curves seg- 
regated according to torpedo type (Figures 1 
to 4), indicates that although the character of 
the noise from all types is roughly similar, 
quite wide variations exist in the intensity of 
the noise not only with different types of tor- 
pedoes but also between individual runs of the 
same type of torpedo. The maximum intensity 
for all types occurs in the region between 300 
c and 800 c. Phgure 5 shows the pressure spec- 



Figure 5. Limits of torpedo noise at 1,000 yards 
for all types measured. 


trum level at 1,000 yards for all types tested. 
These tests are discussed in detail in available 
reports.^’^ 

13 3 SHIP SELF-NOISE MEASUREMENTS 

Merchant Ship Noise 

Measurements of merchant ship self-noise 
were made on a modern tanker of approxi- 
mately 19,000 tons displacement having a 
single screw driven by a steam turbine through 
double reduction gears and capable of speeds 
up to about 18 knots. A photograph of this 
vessel is shown in Figure 12. Twelve pickup 



Figure 6. Variation of self-noise of SS Colorado 
with propeller speed. 



Figure 7. Variation of self-noise of SS Colorado 
at 17 knots with type and location of pickup units, 
compared with U.S. Mark XIV torpedo noise. 


10 


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LSTERN 
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ELECTRIC ; 
DRAFT 

I 1 

3A PICK- 

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ELECTF 

RCK-UI 

W-PROTEC 
P, 20 FOOT 

TIVE--'^ 

DRAFT 


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ELECTRO -PROTEC 
PICK-UP. 24 FOOl 
DRAFT 

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STERN 
FOOT 6 

ELECTRIC 
INCH DRJ 

5A PICK- 
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100 


1000 2 5 10,000 2 

FREOUENCY-CPS 


Figure 8. Comparison of average self-noise of 
SS Colorado at 70 rpm for 20-foot, 0-inch and 24- 
foot, 6-inch drafts. 


SHIP SELF-NOISE MEASUREMENTS 


157 


units were installed at locations inside and just 
outside the hull of this ship. These units were 
of four different types. Types AX58 and JO 
hydrophones (Brush Development Company) 
were externally mounted; Type 5 A (Bell Tele- 
phone Laboratories), and modified VP - 5 
(Brush) vibration pickups were mounted in- 
side the hull. The inside-mounted units meas- 
ured vibration of the ship’s plates. Sensitivity 
and pattern calibrations of these units may be 
found in Division 6, Volume 11. 

Self-noise measurements were made in vari- 
ous frequency bands by means of the 12 pick- 
up units while the ship was running on an 
even keel at a 24.5-foot draft at each of four 
speeds: 26, 50, 70, and 86 rpm, corresponding 
approximately to water speeds of 5, 10, 14, and 
17 knots. In addition, measurements were made 
with the ship ballasted at an even keel draft 
of 20 feet and running at 70 rpm. 

Using these noise data, curves were plotted 
showing the frequency distribution of the self- 
noise in the range from 500 to 25,000 c. Com- 
binations of these curves indicate the self-noise 
variation with speed of the ship (Figure 6), 
with type and location of pickup unit (Figure 
7), and with change of draft (Figure 8). The 
self-noise values are given in terms of pressure 
spectrum levels referred to 1 dyne per sq cm 
for sound in the water (by reference to the 
acoustic calibrations mentioned above) and 
thus are directly comparable to the measure- 
ments of torpedo noise. 



Figure 9. Self-noise as measured by the JK trans- 
ducer in dome on the USS Semmes. 


The measurements indicate that the effect 
of increase in speed of the ship is confined 
mainly to an increase in the higher (above 2 
kc) frequency components of the self-noise. 
Comparison of the self-noise spectra with the 
torpedo noise spectra indicates that, with a 
nondirectional device, the optimum listening 
range of frequencies for torpedo sounds is in 
the region below 3 kc. The self-noise is greater, 
particularly at the higher frequencies, for a 
shallower draft of the ship. No significant dif- 
ference was detected with change in the longi- 
tudinal position of the pickup unit from mid- 
ships forward to the cofferdam section. The 
technique and results of the self-noise measure- 
ments on the merchant ship are discussed in 
detail in reference 2. 

Destroyer Noise in Streamline Domes 

Measurements were made of the noise in the 
50-inch QBF and 100-inch JK domes on the 
USS Semmes, a four-stack destroyer assigned 
to experimental work. This vessel has twin 
screws driven through reduction gears by high- 
and low-pressure steam turbines. Two series 
of measurements were made, the first utilizing 
the QBF and JK transducers as hydrophones, 
and the second utilizing a Type 3A hydrophone 
(Bell Telephone Laboratories) mounted suc- 
cessively in each of the two domes. In each 
case the speed of the ship was varied in ap- 
proximately 5-knot steps from 0 to 25 knots. 



Figure 10. Self-noise vs speed as measured by 
BTL 3 A hydrophone in QBF dome of USS Semmes. 




158 


TORPEDO DETECTION STUDIES AND SYSTEMS 


Curves (Figures 9 and 10) indicate the fre- 
quency distribution of the noise in the range 
from 1 to 30 kc. The measurements, made 
using the QBF and JK transducers, have not 
been compensated to eliminate the effects of 
directionality of these units at the higher fre- 
quencies. Thus, they are not directly com- 
parable with those made using the 3A hydro- 
phone which is virtually nondirectional in the 
frequency region investigated, but the meas- 
ured differences are approximately as ex- 
pected. The pressure spectrum levels fall 
roughly 6 db per octave for the nondirectional 
3A pickup and 12 db per octave for the JK 
hydrophone. 

In general, these measurements^’® indicate 
that the noise in the QBF dome is very nearly 
constant with speed throughout the 1- to 30-kc 
frequency range, up to. approximately 10 knots, 
and rises at the rate of about 2 db per knot 
above that speed. In the JK dome the noise 
increases appreciably before a speed of 10 knots 
is reached, but rises only about IV 2 db per knot 
above that speed. 

Modified Through- the- Hull 
Equipment 

The modified throng h-the-hull equipment ivas 
designed to provide a system capable of de- 
tecting the noise of approaching torpedoes and 
of determining accurately their relative bear- 
ing from the target vessel. It includes an 18- 
inch magnetostriction line hydrophone, a power 
training mechanism, and a cathode-ray tube 
indicator. The hydrophone shaft rotates con- 
tinuously at 60 rpm. The CRO uses a circular 
sweep synchronized tvith the hydrophone rota- 
tion. A target is indicated by brightening and 
increased radial displacement of the trace. The 
acoustic response of the system tvas limited to 
the 20-kc to 30-kc band tvhere the 18-inch hy- 
drophone is fairly directional. Tests from a 
small ship laying to shoived that torpedoes, 
detected on firing, can be followed approxi- 
mately 2,000 yards beyond the hydrophone. 
This equipment, developed by CUDWR-NLL, is 
based on the JP throug h-the-hull equipment 
and a British continuously rotated hydrophone 
in a streamline dome. 



Figure 11. Modified through-the-hull interior 
equipment. 


4 DETECTION EQUIPMENT 

The performance of a continuously rotated 
torpedo detection system modified from 
through-the-hull listening gear, described in 
Chapter 7, was investigated on a small labo- 
ratory vessel. The equipment was modified by 
replacing the hand training mechanism with 
an electric motor and gear reduction unit iso- 
lated from the hull and hydrophone shaft by 
flexible mounts and couplings. The resulting 
rotational speed of the hydrophone shaft was 
60 rpm and the entire assembly was free of 
any significant mechanical noise. This mech- 
anism is shown in Figure 11. 

The 3-foot straight magnetostriction hydro- 
phone, normally used with the through-the- 
hull gear, was replaced by an 18-inch unit of 
the same type capable of being installed in a 
standard 19-inch dome. The electric con- 
nections from the hydrophone were made by 
means of gold-plated slip rings on the rotating 
shaft and gold-plated copper braid brushes 
which gave quiet operation both electrically 
and mechanically. The response of the system 
was limited by filters to a frequency band 10 kc 
wide which could be translated continuously, 
by means of a heterodyning system, through 
the range from 5 kc to 100 kc. In addition, 


DETECTION EQUIPMENT 


159 


some tests were made using a wide-band sonic 
amplifier with high-pass filters having cutoffs 
at 600 c and 2 kc. 

The effects of acoustic signals reaching the 
hydrophone were exhibited on a long-persist- 
ence type screen of a cathode-ray tube using 
a circular sweep synchronized with the rotation 
of the hydrophone shaft. In the absence of 
targets, the trace on the cathode-ray oscillo- 
scope [CRO] was essentially circular except for 
own-ship’s propeller noise indications close to 
180 degrees while the listening ship was under- 
way. A torpedo or ship signal was indicated 
by a brightening and a greater radial displace- 
ment of the trace on the relative bearing of the 
target. 

Tests and Observations 

Only one trial was made of the response of 
the rotating detection equipment to actual 
torpedo signals. As no dome was installed over 
the hydrophone during these tests, it was not 
possible to make observations with the listen- 
ing ship underway. Several firings of 30-knot 
U. S. Mark XIII (aircraft type) torpedoes 
were observed. With the laboratory ship laying 
to (with engines and generators running) near 
the torpedo course at a distance of about 1,000 
yards from the firing barge, it was qualita- 
tively observed that the frequency band from 22 
to 32 kc gave about optimum balance between 
deflection amplitude and sharpness of bear- 
ing for this type of torpedo. In this frequency 
range, the torpedoes were detected immediately 
on firing and could be followed until they had 
traveled slightly more than 2,000 yards beyond 
the listening ship. Calibration of the system 
indicated that V^-inch CRO deflections, ob- 
tained for torpedoes at 1,000 yards, corre- 
sponded to a pressure spectrum level of ap- 
proximately — 62 to —59 db vs 1 dyne per sq 
cm, which checks the levels measured for this 
type of torpedo. Further observations made 
under the same listening ship conditions indi- 
cated that, although greater ranges could be ob- 
tained at lower frequencies, bearing indications 
became increasingly broader with decreasing 
frequency, but it is very doubtful that any range 
advantage would have been found at the lower 
frequencies with the listening ship underway. 


The development time available was insuf- 
ficient to permit a more thorough investigation 
of the optimum frequency range or to deter- 
mine directly the effects of underway noise of 
the listening ship with a dome over the hydro- 
phone and of different rotational speeds of the 
hydrophone upon torpedo detection ranges. 

The equipment modifications and the tech- 
nique and results of the tests are described more 
fully in reference 6. 



Figure 12. The SS Colorado used for tests of E-P 
equipment and for merchant ship self-noise meas- 
urements. 


Electro- Protective [E-P] Torpedo 
Detector 

The Electro-Protective [E-P] Corporation 
torpedo detector is a device used on merchant 
vessels to detect the noise of torpedoes and to 
indicate whether the torpedoes are approach- 
ing from port or starboard. The equipment con- 
sists of tivo nondirectional hijdrophones and an 
electronic indicator mechanism. The hydro- 
phones, installed inside the hull on each side of 
the ship, are modified vibration pickups, sensi- 
tive to sounds from 1,500 c to 3,000 c. The indi- 
cator mechanism is actuated by signals from 
either of the tivo hydrophones. When the sound 
from one of the pickups is sufficiently intense, 
an indicator light and a loudspeaker provide a 
visible and an audible alarm. In tests of this 
device conducted by CUDWR-NLL, detection 
ranges of up to 2,000 yards were obtained in 
moderate seas. 


160 


TORPEDO DETECTION STUDIES AND SYSTEMS 


The E-P torpedo detector is desired to pro- 
vide a means of warning a merchant ship when 
a torpedo is in the vicinity and to indicate from 
which side, port or starboard, it may be ap- 
proaching. This is accomplished by using two 
hydrophone units, one on each side of the ship, 
and feeding their outputs into an electric 
circuit which utilizes an increase in the output 
of one of the units to operate a port or star- 
board indicator light and an audible alarm. 
The hydrophone units consist of modified Type 
VP-5 crystal vibration units (Brush Develop- 
ment Company), and are mounted inside the 
hull near the bow and well below the water- 
line. The two units are transformer-coupled to 
the amplifier indicator rack shown in Figure 
13 and located in the wheel house. 

Circuit Analysis 

As indicated in the schematic diagram. 
Figure 14, two channels are provided, one sup- 
plied by the port and the other by the star- 
board hydrophone. Each comprises a three- 
stage audio-amplifier circuit (V-101, V-102, 
V-103 for the port hydrophone; V-109, V-110, 
V-111 for the starboard), and an alarm circuit 
consisting of a channel section (V-104, V-105, 
V-112, and V-113), and a common section. A 
relay in each channel (K-101 and K-102), re- 
ferred to as the channel relay, is actuated by 
the plate current in the corresponding relay 
control tube, V-105 (port), which has a milli- 
ammeter, M-101, inserted in the cathode circuit 
to indicate this plate current. The channel 
relay is caused to close whenever the signal 
level becomes high enough to give a plate cur- 
rent of approximately 4.5 milliamperes. Clos- 
ing of this relay partially completes the circuit 
including the channel alarm lamps (port, red; 
starboard, green) and starts the process which 
actuates the alarm relay (K-103). The alarm 
relay is controlled by the relay control tube, 
V-114. With the closing of the channel relay the 
grids of V-114 are connected to the cathode- 
bias circuit through R-146, which taken with 
the grid condenser, C-139, forms a time delay 
network. A delay of from 0 to 5 seconds, de- 
pending on the setting of R-146, is thus pro- 
vided, after which the plate current reaches a 
sufficient value to actuate the relay. Closing 



Figure 13. Amplifier-indicator rack of the Electro- 
Protective torpedo detection equipment. 


DETECTION EQUIPMENT 


161 



1 

nnnp 

il'iiiS 

a 

I 5* 

nioin 

1 


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3 a r 

bJ 


n 1 

1 


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Figure 14. Circuit diagram of E-P torpedo detector 


162 


TORPEDO DETECTION STUDIES AND SYSTEMS 


of this relay completes the circuit to the alarm 
lamp corresponding to the particular channel 
receiving the signal and also connects the voice 
coil of the loudspeaker to the output trans- 
former, T-103, to provide an audible alarm. 
The frequency response of the amplifier is 
shown in Figure 15. 



Figure 15. Frequency response characteristics of 
Electro-Protective amplifier for various channel 
sensitivities. Pickup unit and connecting cable are 
used as generator impedance. 

The frequency and directional response char- 
acteristics of the pickup units were measured 
by projecting tones in the water from an un- 
derwater loudspeaker mounted on an auxiliary 
ship. The frequency response data for these 
units indicate that the effective response of this 
type of pickup (Figure 16), coupled to the 
ship’s plates, lies in the region between about 
1,500 and 3,000 c. The directional response data 



FREQUENCY- CPS 


Figure 16. Average response of Electro-Protective 
pickup units as installed on SS Colorado. 


indicate that the unit, as normally installed, 
covers a horizontal angle of about 150 degrees, 
from about 15 degrees off the bow to about 15 
degrees forward of the stern. 

Field Performance 

Checks of the field performance of the tor- 
pedo detector were made at Newport, Rhode 
Island, under conditions closely simulating an 
actual attack. Six U. S. Mark XXIII (air-steam 
drive) torpedoes were fired at 45 knots from 
a distance of 2,000 to 3,000 yards with the 
tanker underway at approximately 17 knots 
and at a 24.5-foot draft. 

With the ship underway, the detection 
ranges were found to vary from about 700 to 
1,700 yards, corresponding to from 28 to 68 
seconds of warning time. These observed 
ranges are in substantial agreement with those 
which would be anticipated on the basis of the 
separately measured torpedo and self-noise val- 
ues and consideration of the alarm sensitivity 
settings used during the trials. From the 
range data on the Mark XXIII torpedo runs, it 
is possible to approximate the warning times 
to be expected with the E-P device for other 
types of torpedoes. For the case of the tanker 
running at approximately 17 knots and at a 
24.5-foot draft, these probable warning times 
are given below. 


Torpedo Type 

Speed 

(knots) 

Warning 

(seconds) 

Detection 

Range 

(yards) 

U. S. Mark XVIII 

(electric) 

30 

15-38 

250-630 

British Mark VIII 
(semidiesel) 

45 

20-48 

500-1,200 

German G-7-e 

(electric) 

27 

17-25 

250-375 


These calculated ranges are based on opera- 
tion of the detector with the same alarm sen- 
sitivity settings as those used during the trial 
runs which were considerably higher than nor- 
mally recommended. 

Laboratory Tests 

A disk sound recording of ship noise, taken 
through the E-P pickup on a tanker very simi- 
lar to the one used for the field tests, and a 
recording (made utilizing a frequency band 


C( 


CONCLUSIONS 


163 


corresponding to that of the E-P pickup) of 
the noise of an approaching Mark XXIII tor- 
pedo were played into the input of the torpedo 
detector both individually and simultaneously 
at various levels. By varying the relative 
amounts of torpedo and ship noise fed into the 
E-P equipment, and by changing the sensitivi- 
ty and time delay settings, different conditions 
of operation were closely simulated. The results 
of this procedure indicated that a definite ad- 
vantage in detection time could be obtained by 
keeping the sensitivity of the system as high 
as possible considering ship self-noise alone 
and setting the alarm time delay sufficiently 
long to minimize false alarms. 

CONCLUSIONS 

From a consideration of the results of the 
investigations carried out as outlined above, a 
number of broad conclusions were reached: 

1. The sonic detection of submarine torpe- 
does at ranges up to the order of 2,000 yards 
from fast-moving merchant ships appears 
feasible under moderate sea conditions with 
either the rotating directional or the non- 
directional type of detection system investi- 
gated. However, considerable care in installa- 
tion, adjustment, and use is necessary if such 
ranges are to be consistently obtained. Careful 
monitoring of the nondirectional system is 
essential and the directional system requires 
a constant attentive watch. 

2. Improvement in the performance of the 
nondirectional (E-P) system is necessary to 
make it satisfactory for routine use on fast 
merchant vessels. Specific suggestions for pos- 
sible improvements to this device include: (a) 
the use of closer limits on the alarm sensitivity 
than are now recommended, (b) the use of a 
gain control on the speaker to permit better 
aural monitoring, (c) changes in the circuit 
which would improve its response to transient 
peaks, (d) redesign of the alarm circuit to pro- 
vide actuation on modulation products instead 
of straight intensity discrimination and, (e) 
improvements to the pickup which would broad- 
en its response to include the region between 
500 c and 1,500 c. As emphasized in the refer- 
ence, however, it is not believed that these sug- 
gested possibilities have been explored com- 


pletely enough to justify their being considered 
as recommendations. 

3. Tests of the particular rotating direc- 
tional gear developed at the laboratory during 
these investigations were insufficient in them- 
selves to determine completely the capabilities 
of this type of system. However, a considera- 
tion of the separately )neasured ship self-noise 
(with directional receivers) and the torpedo 
noise, coupled with the experience of the 
British with similar types of detection systems 
indicates that this type of detection equipment, 
in optimum adjustment, should give good 
results. 

4. The advantages of the directional method 
over the nondirectional (actually, bidirection- 
al) E-P system are: (1) accurate determination 
of target bearing and (2) possibly greater de- 
tection ranges. The disadvantages are: (1) 
greater complexity of the equipment required, 
(2) considerably greater installation problems, 
and (3) the necessity for constant monitoring. 



Figure 17. Torpedo detection modification [TDM]. 


WCA-2 Torpedo Detection 
Modification^ 

The torpedo detection modification [TDM] 
to submarine supersonic equipment ivas de- 
veloped to provide for the sonic detection of 

^ A similar modification employing cathode-ray 
oscilloscope [CRO] indication was incorporated in the 
ynine and torpedo detection [MATD] equipment for 
WCA-2 gear and is described in Division 6, Volume 15. 




164 


TORPEDO DETECTION STUDIES AND SYSTEMS 


torpedoes from submarines cruising on the 
surface, particularly to and from patrol areas. 
The modification is specifically applied to the 
submarine WCA-2 equipment but is adaptable 
to any similar type of gear. The equipment in- 
cludes a slip-ring mechanism to enable con- 
tinuous rotation of the projector at 12^2 rpm, 
a speed faster than that used for normal listen- 
ing or echo-ranging , and incorporates a re- 
corder to give visual bearing indications. At a 
speed not exceeding 12 knots, submarines can 
usually detect and determine the relative bear- 
ing of enemy torpedoes at ranges of up to about 
3,000 yards with this equipment. The tor- 
pedo detection modification was developed by 
CUDWR-NLL. 

136 INTRODUCTION 

It was believed that an acoustic system could 
be devised by means of which a submarine run- 
ning on the surface could detect and determine 
the relative bearing of torpedoes fired at it in 
time to take successful evasive action. A survey 
of the characteristics of the transducers and 
amplifiers used with the current submarine 
supersonic gear indicated that this equipment, 
with suitable modifications to other components 
of the system, would be adaptable to use for 
torpedo detection. 

Consequently a development program was 
undertaken, directed toward making the neces- 
sary modifications to provide an effective sys- 
tem as quickly as possible and toward evaluat- 
ing the torpedo detection capabilities of such 
a system. As a result of this work, modifica- 
tions have been evolved which are specifically 
applied to the WCA-2 sonar system,^ but whose 
essential characteristics are applicable to any 
similar type of gear. Modifications include pro- 
visions for (1) continuous rotation of the pro- 
jector in one direction, (2) rotation at a more 
rapid rate, and (3) incorporation of a chemical 
recorder to supplement aural listening and to 
provide a permanent record of the traces. 

Tests of this system indicate that for sub- 
marine speeds not exceeding 12 knots, torpedo 
sounds can usually be detected, and the relative 
bearings determined, as far away as 3,000 
yards. Later tests indicate the desirability of 


providing a streamline dome over the trans- 
ducer to increase detection ranges and reduce 
mechanical strain on the training system at 
submarine speeds in excess of 10 or 12 knots. 

13.7 SURVEY OF THE PROBLEM 

A primary consideration in the development 
of acoustic equipment for the detection of tor- 
pedoes from submarines was to make such 
equipment available as quickly as possible. For 
this reason, and because the space for new 
equipment aboard modern submarines is very 
limited, it was necessary that maximum use be 
made of existing submarine sonar equipment. 

To be most effective, a torpedo detection sys- 
tem for use on submarines should not only have 
as long a detection range as possible but, be- 
cause the type of evasive action may depend 
considerably on the sector from which the at- 
tack is made, the system should also give an 
accurate indication of the torpedo’s bearing. 

Consideration of previous work, discussed 
in connection with the problem of sonic detec- 
tion of torpedoes from merchant vessels, to- 
gether with a knowledge of the characteristics 
of existing submarine supersonic listening and 
echo-ranging equipment, indicated that these 
requirements could be most readily met by 
modifications to this equipment. In the widely 
used WCA-2 sonar equipment, the QB projec- 
tor is rotated at maximum speed of 4 rpm for 
normal listening and echo-ranging operations 
and, due to a cable connection, its rotation is 
restricted to approximately 2 turns in one 
direction. 

Because of the need for detecting torpedoes 
as soon as possible after firing to allow the 
maximum time for evasive maneuvers, the ro- 
tational speed of the projector should be in- 
creased to reduce the 15-second interval be- 
tween successive explorations at a given 
bearing and to aid the sound operator in dif- 
ferentiating between background noise and low- 
level signals from distant torpedoes. An upper 
limit on the rotational speed is determined by 
the necessity for the operator to correlate 
changes in sound level with the projector bear- 
ing, as indicated by the rotating pointer of 
the bearing repeater and by the mechanical 


DEVELOPMENT 


165 


limitations of the training mechanism. A speed 
of 121/2 1‘Piii, providing successive observations 
on a given bearing approximately once every 
5 seconds, satisfies all the requirements reason- 
ably well. Because of the faster rotational 
speed of the projector and in order to simplify 
other features of the system, it was necessary to 
provide for continuous rotation in one direction 
by elimination of the cable connection and limit 
switches. 

The background noise occurring on bearings 
abaft the beam increases rapidly with speed for 
submarine speeds in excess of 10 knots and this 
is believed due to turbulence or cavitation about 
the projector. For this reason and in order to 
reduce mechanical strain on the training mech- 
anism, it is desirable to provide a streamline 
housing for the projector. 

A change in the character or a very small 
change in the intensity of the received sound 
occurring as a rotating directional receiver 
passes rapidly through a particular bearing 
can usually be more readily detected aurally 
than by other means. For this reason and be- 
cause experience may enable an operator to dis- 
tinguish a difference between the noise from a 
torpedo and that from other types of targets, 
it is believed that aural listening provides the 
best primary means for long-range detection. 
It is desirable, however, to provide a recorder 
or other visual indicator as a supplement to 
aural monitoring. Such an indicator, although 
not usually so sensitive as the ear to signals 
very close to background level, is likely to yield 
more accurate bearing information and, in the 
case of a recorder, provides a permanent rec- 
ord which is useful in keeping track of any 
changes of target bearing between successive 
swings of the projector. 

DEVELOPMENT 

The development work on this problem was 
essentially that of determining the best and 
most expeditious means of modifying the 
WCA-2 equipment to meet the requirements of 
a torpedo detection system, as outlined in the 
preceding section, and of evaluating the per- 
formance of the system. 


Continuous Rotation of Projector®-® 

In order to make continuous rotation of the 
QB projector possible, it is necessary to provide 
electric connections to carry the received signals 
from the rotating to the stationary elements of 
the system without mechanically restricting ro- 
tation of the shaft. To accomplish this, the 
flexible cable connection to the projector as- 
sembly is replaced by a slip ring and collector 
braid mechanism. 

The slip-ring assembly, shown in Figure 18, 
consists of two conductor rings, having outside 



Figure 18. Slip-ring assembly. 

diameters of 8% inches, held in three hard- 
rubber holder rings. Each conductor and 
holder ring is split on a major diameter to per- 
mit installation around a continuous shaft, and 
the assembly is made rigid by staggering the 
joints between the split-ring sections. Each 
conductor ring is a lamination consisting of a 
rolled bronze channel, faced with a gold alloy 
overlay and sweat-soldered to a brass backing 
ring. 

In the collector mechanism, two gold-plated 
copper braids are used to make electric con- 
tacts with the slip rings. These are secured to 
hooks fastened to stationary collector blocks 
which form a termination for a cable leading 
to other components of the system. The proper 
tension of the braids is maintained by means 
of helical springs. The slip ring and collector 
block installation is shown in Figure 19. 

With elimination of the necessity for a cable 
connection to the projector assembly, the need 
for limit switches to restrict its rotation is re- 
moved, and performance of the system is im- 




166 


TORPEDO DETECTION STUDIES AND SYSTEMS 


proved in normal listening or echo-ranging op- 
erations as well as for the torpedo detection 
application. 



Figure 19. Slip ring and collector block installation. 


Increased Speed of Rotation 

Preliminary tests conducted at sea indicated 
that the projector may be rotated at a speed 
of 121/2 ppm without damage to the existing 
training motor due to overloading. Rotation 
at this speed is accomplished by replacing the 
existing 66-to-l reduction gear with a 20-to-l 
gear. In order to permit operation of the shaft 
at the original maximum of 4 rpm when the 
projector is used in normal listening or echo- 
ranging operations, the resistor values in the 
QB remote control unit (in the conning tower) 
are raised so that, on the first four contacts, 
the approximate shaft speeds are successively 
1.0, 1.6, 2.4, and 4.0 rpm. A change from these 
normal speeds to the scanning speed of 12 1/2 
rpm is effected by using the fifth position of 
the remote control unit handle, in which posi- 
tion the resistances are out of the circuit. In 
the fifth position a spring latch, attached to the 
slewing control handle, fits over a stud fastened 
to the door of the control unit and holds the 
handle in place. This latch and stud also serve 
as a stop to prevent inadvertent use of the 
scanning speed during normal listening or 
echo-ranging operations. 

When the local control unit (in the forward 
torpedo room) is used, provision is made for 
normal shaft speeds by the introduction of an 
additional 1,500 ohms in series with the MG 
generator field. This additional resistance is 
thrown into the circuit when the transfer 
switch, used for selecting local training control 


of the WCA-2 starboard (QB) or port (JK- 
QC) projectors, is, in the starboard-train posi- 
tion. No provision is made to permit operation 
of the QB projector at scanning speeds when 
the system is controlled locally, but in an emer- 
gency this can be done by shorting out the 
added 1,500-ohm resistor and securing the con- 
trol handle in its extreme right-hand position. 

Visual Indicator 

To provide a visual indicator as a supplement 
to aural detection of torpedo signals, a Sanga- 
mo sound range recorder, as described in Di- 
vision 6, Chapter 6 of Volume 15, but modified 
to show bearings instead of ranges, is con- 
nected to the output of the WCA-2 heterodyne 
receiving amplifier. Modification of the re- 
corder for this purpose is accomplished by ar- 
ranging for the flyback of the recorder stylus 
to be controlled by a contactor on the projector 
shaft. A switch, keyed by this contactor, causes 
the stylus to return to the left margin of the 
recorder paper during the interval when the 
projector is scanning the sector from approxi- 
mately 165 degrees to 195 degrees relative 
bearing, containing the submarine’s screws. 
The stylus travels from left to right across 
the paper once for each revolution of the pro- 
jector. When a target signal is being received, 
an increased flow of electric current from the 
stylus to a metal roller beneath the moist, 
chemically treated, recorder paper causes a 
darkening of the trace at the location on the 
paper corresponding to the relative bearing of 
the target. In Figure 20 the bearing recorder is 
shown as installed, together with other com- 
ponents of the system, in the conning tower. 

13.9 TESTS 

The torpedo detection modifications outlined 
in the preceding sections were carried out on 
the WCA-2 equipment of a fleet submarine and 
the installation was tested at sea under simu- 
lated operating conditions. Arrangements were 
made for a second submarine to fire a number 
of torpedoes at the boat containing the modified 
equipment while the latter boat was cruising 
on the surface at a speed of 8 knots. Each of 
these torpedoes, which were fired at ranges 
varying from about 3,000 yards to over 5,000 


TESTS 


167 


yards, were detected, both aurally and by the 
visual indicator, immediately after firing, and a 
continuous record of the bearing of each was 
obtained on the recorder from the time of fir- 



Figure 20. Bearing recorder and other system 
components. 


ing until the torpedoes had passed under the 
submarine. A typical recorder tape for such 
a run is shown in Figure .21. 

Subsequent measurements of self-noise on 
this submarine at speeds of 8, 12, and 15 knots 
indicated that detection of torpedoes at ranges 
of 3,000 yards or greater can be expected only 
for ship speeds not exceeding 12 knots. At 15 
knots the self-noise, particularly for bearings 
abaft the beam, was ..found to have increased 
sufficiently to reduce the expected detection 
ranges to about 1,5|(0 yards. Later tests of a 
similar nature invalving three other fleet sub- 
marines with modified WCA-2 equipments and 
conducted at ship speeds of 8 to 15 knots sub- 
stantially confirmed these results. 

Tests were also made to determine the loads 
on the training motor when the QB projector 
was rotated at the scanning speed of 121/2 rpm 
with the submarine cruising on the surface and 
at 4 rpm with the boat underway submerged. It 
was determined that (1) the loads imposed on 
the motor under the conditions of the test did 
not exceed its full rated load, (2) a factor of 
safety is provided because the motor is de- 


signed to carry safely loads up to 175 per cent 
of full load, (3) the temperature rise of the 
equipment when operated continuously at 12i/> 
rpm for 4 hours was well within safe limits. 
From these tests it was concluded that the 
training motor and the motor generator set 
would operate within safe limits under all an- 
ticipated conditions of operation after substitu- 
tion of the 20-to-l reduction gear for the 
original 66-to-l gear. 

Later operational reports from this boat 
and another submarine with similar modified 
WCA-2 equipment indicated some overheating 
of the training motor during continued rota- 
tion of the projector at 12i/^ rpm when the 
submarine was operating on the surface at 
speeds in excess of 10 knots. This overheating 
was not serious enough to disable the equip- 
ment, but because of concern about it and the 
undesirable limitation of detection ranges by 
high noise levels occurring on bearings abaft 
the beam at higher submarine speeds, it was 


RELATIVE BEARING 



Figure 21. Typical torpedo trace as recorded aboard 
target submarine. 


decided to investigate the possible benefit of 
a streamline dome over the projector. 

Projector with Streamline Dome 
Comprehensive tests were consequently made 
before and after installation of a 57-inch 
welded-on dome over the QB projector of an- 
other fleet submarine equipped with modified 
WCA-2 equipment. These tests were designed 
to determine the effects of the dome on self- 
noise (believed due to turbulence or cavitation 




168 


TORPEDO DETECTION STUDIES AND SYSTEMS 


about the projector at the higher submarine 
speeds) and on mechanical strain on the train- 
ing mechanism. Self-noise measurements and 
heat tests were made at submarine speeds rang- 
ing from 5.5 to 20 knots. As a result of 
these tests, the following conclusions were 
reached. 

1. At frequencies of 17, 24, and 30 kc, the 
installation of a streamline dome on the QB 
projector results in a consistent reduction in 
self-noise abaft the beam at all submarine 
speeds in excess of 6 knots. 

2. In the forward sector (ahead of beam) 
use of the dome results in an increase in self- 
noise at the lower speeds. This effect is particu- 
larly apparent at a frequency of 30 kc where 
an increase of self-noise as high as 10 db 
occurs at a speed of 11 knots. At speeds of 15 
to 16 knots and above, however, a progressive 
decrease in self-noise results, and the increase 
of self-noise introduced in the forward sector 
is not expected to reduce detection ranges suf- 
ficiently to impair the satisfactory and useful 
operation of the TDM. 

3. Increases in detection ranges at ship 
speeds of 15 knots and greater, especially abaft 
the beam, are expected to improve the value 
of the TDM to submarines on war patrol. 

4. Use of the dome makes it possible to hoist 
and lower the projector at all ship speeds. 

5. Loads imposed on the training motor, 
with the submarine operated at 15 knots and 
the projector rotated at scanning speed, are 
reduced approximately 200 watts (33% per 
cent) to about 50 per cent of rated full load 
by the use of a dome. 

6. Loads imposed on the MG motor are also 
lower by approximately 200 watts, or about 20 
per cent, with the dome installed. 

7. With the loads encountered, temperature 
rise of the equipment after 5 hours of continu- 
ous operation at scanning speed and at ship 
speeds of 15 knots is well below the rated 40 C 
allowable rise either with or without the use of 
a dome. 

8. Mechanical difficulties sometimes en- 
countered, even at the normal 4-rpm searching 
speed, are largely eliminated through the use 
of a dome, provided the training equipment 
and the hoist-lower mechanism is in good con- 
dition at the time the dome is installed. 


In addition to the self-noise and heat tests 
made with and without the dome, a series of 
torpedoes fired at this submarine while the 
boat was underway at speeds of 8 to 18 knots 
with the dome in place were detected. At sub- 
marine speeds of 12 to 15 knots, detection 
ranges varied from 1,160 to 2,580 yards. At 
the 15-knot speed several ranges of approxi- 
mately 2,000 yards, corresponding to warning 
times of about 1.25 minutes for a 45-knot tor- 
pedo, were obtained. The sea was state 2 to 4, 
according to the scale given in Instructions to 
Marine Meteorological Observers, U. S. Depart- 
ment of Commerce, during these tests. For this 
reason the results are not considered to show 
conclusively the effect of the dome. The tests 
afforded an opportunity, however, for making 
phonograph recordings of torpedo sounds, as 
detected by the modified equipment, for subse- 
quent use in training operators. 

13.10 MODIFICATION KITS 

Performance of the experimental torpedo 
detection modifications to WCA-2 equipment 
was judged by the Navy to be favorable enough 
to warrant production of kits in sufficient 
quantities to incorporate the modifications (ex- 
clusive of the dome) on virtually all fleet 
submarines. 

1311 CONCLUSIONS 

As a result of the extensive tests of the 
torpedo detection modifications to WCA-2 
equipment, it is believed that the gear will pro- 
vide, under most operating conditions and at 
submarine speeds up to 12 knots, reasonably 
adequate warning of the approach of an enemy 
torpedo without the use of a dome over the 
projector. Because of the decreased detection 
ranges on bearings abaft the beam and the in- 
creased load on the training mechanism at 
speeds in excess of 10 or 12 knots, it is recom- 
mended that domes ultimately be supplied for 
installation over the QB projectors. 

It is pointed out that in order to attain satis- 
factory performance, proper monitoring of the 
equipment is essential and the operator must 
give undivided attention to the indicators 
throughout the period of each watch. 


Chapter 14 


HARBOR PROTECTION SYSTEMS 


B efore the development of the cable-con- 
nected hydrophone system and the an- 
chored radio sono buoy [ARSB], magnetic loop 
cables, minefields, submarine nets, and other 
fixed obstructions supplemented by armed pa- 
trol craft formed the protective barriers for in- 
ner harbor areas in wartime. Magnetic loop 
cables detected intruders at the seaward end 
of the harbor, but these cables only register the 
passage of any ferrous vessel crossing the loop 
and do not indicate actual position. 

The cable-connected hydrophone system and 
the ARSB, discussed in this chapter, were pro- 


posed as alternate sonic methods of detecting 
and locating harbor intruders. Since the cable- 
connected hydrophones 'are tripod-mounted and 
require long transmission lines, they cannot be 
installed hurriedly or in deep water. The buoy, 
on the other hand, with its simple structure and 
radio transmission facilities, may be anchored 
relatively quickly in deep harbors. 

Both hydrophone systems provide about the 
same detection range under good conditions, 
but the buoy suffers from instability in rough 
seas, inadequate low-frequency response, and 
higher background noise. 


Cable-Connected Hydrophone System 


The cable-connected hydrophone system ivas 
designed to detect and to determine the approx- 
imate location of enemy submarines approach- 
ing a harbor. The equipment consists of a series 
of tripod-mounted Brush C-37 crystal hydro- 
phones regularly spaced at intervals of 1,000 
yards across the harbor entrance. The hydro- 
phones, each consisting of eight parallel-con- 
nected elements and a built-in step-doivn trans- 
former, are connected by a submarine cable to 
a shore station where a switching mechanism 
and a sonic listening amplifier are provided. 
The switching mechanism, a rotary selector 
switch and relay circuit, automatically selects 
the separate hydrophones for listening, alloiv- 
ing the operator to monitor the various units 
successively . A timing circuit comprising mo- 
tor-driven cams enables the operator to adjust 
the listening time intervals to 2, 3, 5, 7^4, or 
10 seconds. The listening amplifier has a fre- 
quency characteristic uniform within 3 db over 
the range 70 c to 12,000 c. High-pass filters 
with cutoff frequencies of 600 c, 1,200 c, and 
2,U00 c are available. Either headphones or 
loudspeaker may be used for listening with 
this system. Development work on the system 
was done on an advisory basis with the Bureau 
of Ships by the CUDWR-NLL, with the close 
cooperation of BTL. 



eQNFlOENTIA L^ 169 


170 


HARBOR PROTECTION SYSTEMS 


14.1 INTRODUCTION 

The proposal for a system of anchored cable- 
connected hydrophones for harbor protection 
led to the design and installation of two cables. 
The first of these, laid near the entrance to 
Block Island Sound, was an experimental sys- 
tem used primarily to determine the require- 
ments for this application. The second system, 
at Cape Henry, designed on the basis of data 
gathered with the first, served as a model for 
further military installations. 

The Cape Henry installation consisted of 14 
tripod-mounted hydrophones spaced 1,000 yards 
apart along an armored cable beginning at a 
point about 5 miles from shore and terminating 
in a shore listening station. Provision was made 
for shore-controlled automatic switching be- 
tween the hydrophones and for listening by 
means of either headphones or a loudspeaker. 
Although some initial trouble developed in the 
system from hum pickup and minor mechanical 
failures, these difficulties were ultimately cor- 
rected and satisfactory performance of the in- 
stallation was obtained. Listening ranges of 
over 7,000 yards were attained for surface 
ships. Determination of approximate target 
position was possible by comparison of the 
relative intensities of signals received on two 
or more of the spaced hydrophones. 

14.2 DESIGN PROBLEMS 

A number of sonic methods of detection have 
been proposed to supplement the loop cables by 
providing more accurate information concern- 
ing the location of vessels. Among these, echo- 
ranging stations located on the ocean floor, 
rotatable binaural listening systems, arrays of 
ARSB’s, discussed later in the chapter, and 
systems of cable-connected hydrophones re- 
ceived serious consideration. Although more 
accurate information can be obtained with 
echo-ranging or binaural systems than with 
nondirectional buoys or the cable-connected 
hydrophone system, serious mechanical diffi- 
culties are encountered in providing the com- 
plicated underwater mechanisms necessary for 
the former. Cable-connected listening hydro- 
phones appeared to offer the best possibilities 
for use as a secondary detection system for 
harbor protection. 

Qi 


The use of cable-connected hydrophones re- 
quires unit spacing to insure overlapping of 
effective detection areas and careful selection 
of locations where favorable listening condi- 
tions exist in the seaward direction and along 
the cable. Ambient noise measurements and un- 
derwater-listening-condition surveys in harbors 
have established certain facts that restrict the 
choice of locations. These studies have shown 
that transmission of sound is poor over steep 
submarine valleys or shoal spots and over very 
muddy bottoms and that certain localities are 
unsuited because of excessive noise levels which 
may result from any of several sources. Among 
these are (1) noise-producing fish and other 
forms of marine life, notably croakers and 
snapping shrimp, (2) bottoms composed of 
loose stones, especially where strong tides or 
heavy seas cause frequent shifting, and (3) 
manufacturing, shipbuilding, and other similar 
activities on or near shore. 

The character of the underwater sound re- 
ceived from a ship provides an experienced 
listener with considerable information concern- 
ing the type and speed of the vessel. When a 
number of spaced hydrophones are available 
for successive listening, the relative magnitude 
of the sound intensity received by two or more 
loose stones, especially where strong tides or 
crease in intensity provide additional informa- 
tion concerning the vessel’s location and direc- 
tion of travel. 

In order to make full use of these potential 
sources of information, the cable-connected 
hydrophone system should provide high-quality 
reproduction at the listening station, well-bal- 
anced sensitivity among the various channels, 
and freedom from noise interference. These re- 
quirements necessitate (1) hydrophoneshaving 
reasonably flat frequency response character- 
istics and uniform sensitivity high enough to 
insure that unavoidable water background 
noise, not inherent cable noise, limits the range, 
(2) a cable whose transmission characteristics 
do not seriously alter the character or relative 
levels of the acoustic signals and which is not 
exposed to serious electric interference, (3) a 
high-fldelity amplifler and reproducer, and (4) 
shore-controlled switching between the outputs 
of the individual hydrophones. 







THE BLOCK ISLAND EXPERIMENTAL CABLE SYSTEM 


171 


In addition, because selective listening may 
sometimes be desirable in discriminating 
against certain types of ambient noise, suitable 
filters should be provided in the amplifier. The 
sonic noise from croakers and most fish ^ ^ lies 
largely in the frequency range below 2 kc, while 
that from snapping shrimp is almost entirely 
above 1 kc.^ Provision of a high-pass filter with 
a cutoff at 2 kc, therefore, permits listening to 
ship sounds in areas where fish are prevalent 
while a low-pass filter having a 1-kc cutoff dis- 
criminates against the noise of snapping 
shrimp. 


14.3 qjij; BLOCK ISLAND 

EXPERIMENTAL CABLE SYSTEM 

Objectives 

Design and installation of an experimental 
system in Block Island Sound ^ were undertaken 
as a means of obtaining information on a num- 
ber of questions pertinent to the successful 
operation of a cable-connected hydrophone sys- 
tem. It was desired to (1) make tests to deter- 
mine the usefulness of cable-connected hydro- 
phone listening under operating conditions, (2) 
obtain information that would indicate the 
most suitable types of hydrophones, ampli- 
fiers, and headphones or loudspeaker, (3) 
determine the best available methods of instal- 
lation for the hydrophones and other under- 
water components, (4) investigate designs for 
underwater cable junction boxes and the re- 
liability of vacuum-type relays for use in such 
boxes, (5) determine the electric loading of the 
cable necessary for optimum transmission char- 
acteristics, and (6) obtain sufficient operating 
experience to enable recommendations for 
methods of observation. 

In addition to the information expected from 
the experimental cable installation, underwater 
sound surveys were undertaken to determine 
listening conditions in several areas proposed 
for tactical installations. 

Description 

The Block Island hydrophone system utilized 
two armored quadded cables 8,000 and 10,000 
yards long. Each cable was spliced at 1,000- 


yard intervals and the larger cable was pro- 
vided with loading coils at each splice. Four 
hydrophone stations, at respective distances of 
7,000, 8,000, 9,000, and 10,000 yards from 
shore, were served by the two cables. A single 
tripod provided at each station served as a 
mounting for two different types of hydro- 
phones. Terminal boxesl mounted on each tripod 
contained the hydrophone-to-cable splices, cou- 
pling condensers, and glass-enclosed contact 
magnetic relays used to switch from one hydro- 
phone to another. 

Cable Loading, The loading coils used at 
1,000-yard intervals in the longer cable were 
of the “wedding ring” type (Western Electric 
Type No. 0-162490), designed to load the cable 
for a nominal cutoff frequency of 15,700 c and 
an effective transmission band of 12,500 c. 
“Straight-through” and Y-type nonwatertight 
splice cases were used to permit continuity in 
the mechanical protection of the cable. Each 
conductor was made watertight individually at 
the splices by means of several layers of DR 
tape. 

Tripods. The hydrophone tripods, 8 feet high 
and 7 feet wide at the base, were constructed 
of 1-inch steel tubing set into 100-pound con- 
crete blocks at the base of each leg. Six hun- 
dred-foot lateral cables connecting the hydro- 
phones to the main cables through Y-splices 
permitted the tripods to be raised separately 
for maintenance purposes. All flexible parts 
were secured to the tripods to prevent water 
currents from causing mechanical noise. 

Hydrophones. The eight vertically-mounted 
hydrophones installed on the four tripods com- 
prised two 4-foot and two 6-foot magnetostric- 
tion units and three 16-inch Brush C-23 and 
one 54-inch Brush C-37 crystal units. Two units 
were served by each of the four pairs of wires 
provided in the two main cables. Selection of 
the desired hydrophone of the two on each pair 
was accomplished by actuation of the relays at 
the tripods by compositing (CX) circuits 
similar to those used in standard telephone 
practice. 

Shore Receiver. At the shore station a pre- 
amplifier, intermediate amplifier, and power 
amplifier were utilized to provide for listening 
by means of a 12-inch speaker or high-quality 
headphones. 




172 


HARBOR PROTECTION SYSTEMS 


Hydrophones. Extensive tests were con- 
ducted to determine the effectiveness of this 
type of hydrophone installation as an integral 
part of a harbor-defense system and to obtain 
the information necessary to evaluate the char- 
acteristics, performance, and reliability of the 
various system components. Listening tests, in 
which submarines were maneuvered in the 
hydrophone cable area, resulted in minimum 
detection ranges of from 1,000 to 2,000 yards 
with the boats operating at 80 rpm (4 knots) 
at periscope depth. Under more extreme evasive 
conditions, with the boats operating submerged 
at creeping speeds of 2 knots (40 rpm) , detec- 
tion was still possible, principally as a result of 
the sounds from various auxiliary machinery 
within the boats. These tests served also as a 
means of comparing the performance of the 
various types of hydrophones. It was found that 
in this application the 54-inch long Brush C-37 
crystal unit gave the best balance between sen- 
sitivity, frequency characteristic, and direc- 
tional exclusion of surface noise. 

Recorders. During routine listening tests ex- 
tending over a period of several months, an 
investigation was made of graphic recorders 
and phonograph recorders as detection aids to 
supplement listening. It was found that in some 
cases small changes of level could be detected 
more readily with a linear graphic level re- 
corder (an Esterline-Angus recording ammeter 
with a linear rectifier) than by ear, although 
the listening differential generally favors aural 
methods of detecting below-background noises, 
particularly when discrete frequency compo- 
nents are present. The graphic recorder, fur- 
nishing a permanent time record of ship move- 
ments, also makes possible the identification of 
various types of vessels from their “signature’' 
on the tape. For these reasons it was considered 
a useful addition to the hydrophone monitoring 
equipment. Tests also showed that a magnetic- 
tape loop recorder is advantageous for listening 
repetitively to sounds of a transient nature. 

Amplifiers. Various types of commercially 
available amplifiers were tried out in a series of 
tests on the Block Island experimental system,^ 
but no single amplifier was found which ade- 
quately met all the requirements for a unit to 
be used with a tactical installation. Conse- 


quently, a special amplifier was designed for 
this purpose. 

Cable. Calculations of the impedance and 
transmission characteristics of the Block Island 
hydrophone cable, based on constants deter- 
mined by measurements on a sample section, 
showed that the estimated improvement of 
transmission by loading could not be realized 
due to a large increase in leakage conductance 
of the cable after prolonged submergence in 
sea water. 

Maximum Ranges. Under normal listening 
conditions large surface ships could be heard 
at ranges of 8 to 15 miles and submarines on 
normal practice maneuvers could at times be 
heard as far away as 8 miles. The character or 
severity of the temperature gradient conditions 
in the water apparently had negligible effect on 
the listening conditions, presumably because 
the shallow water and hard bottom in the area 
provided multiple-path sound transmission. 

Comparison with ARSB. To compare the per- 
formance of the cable-connected hydrophone 
system with that of the ARSB, a buoy was 
anchored in the vicinity of the hydrophone 
cable and provision made for alternate listen- 
ing with the two systems. It was concluded 
that (1) the ARSB and the cable-connected 
hydrophones had about the same detection 
range under good conditions; (2) the deficien- 
cies of 1-f response of the buoy hydrophone 
(Brush C-23) made identification of the vari- 
ous types of vessels difficult; (3) the buoy, 
having its hydrophone closer to the surface, 
picked up more background noise; and (4) 
tangling of the buoy’s hydrophone and battery 
cables caused some rubber squeaks particularly 
in a fast-running tide. 

144 the CAPE HENRY CABLE- 
CONNECTED HYDROPHONE 
INSTALLATION 

The component parts and methods of instal- 
lation used in the Cape Henry cable-connected 
hydrophone system, a tactical installation at 
the entrance to Chesapeake Bay, were chosen 
largely on the basis of the experience gained in 
the operation and testing of the Block Island 
system. 


CAPE HENRY INSTALLATION 


173 


The Cape Henry system incorporates 14 
Brush C-37 hydrophones mounted vertically on 
tripods spaced 1,000 yards apart and connected 
to a shore listening station by means of a 16- 
conductor armored cable having a central quad 
of two listening pairs. Adjacent hydrophones 
are connected to alternate listening pairs while 
switching between hydrophones is accomplished 
by means of relays controlled by the 12 outer 
conductors of the hydrophone cable. Three hun- 


dred-foot, four-conductor, lateral cables are 
used to connect each hydrophone to the main 
cable at 1,000-yard splice intervals. Waterproof 
splice boxes housing 6-inch loading coils are 
provided at each main cable splice. Terminal 
boxes, containing the relays, attenuating pads, 
and the connections between the individual 
hydrophone cables and the lateral cables, are 
mounted on the tripods. A schematic diagram 
of the cable layout is shown in Figure 2. 


I - I9,"Y" SPLICES 



rf ^MBrTifnvr ,rrT . ,i ^^ 


174 


HARBOR PROTECTION SYSTEMS 


Cables. The conducting portion of the main 
cable (Simplex Wire and Cable Company, cable 
Type No. 107) consists of a center quad of 4 
No. 14, 7-strand, tinned copper wires and a 
concentric ring of 12 No. 16, 7-strand (4 cop- 
per, 3 steel) wires. Rubber and thermoplastic 
insulation is provided and mechanical protec- 
tion is furnished by 33 strands of No. 12 steel 
wire and an outer sheath of impregnated jute. 
The lateral cable (Simplex Wire and Cable 
Company, Type No. 108) is similar to the main 
cable, but its conductors consist of only the 
center quad, one pair carrying the signal and 
the other serving the relay control circuit. Fig- 
ures 3 and 4 show sectional views of the two 
cables and include tables of their physical 
properties. 



1. 4-7 STR TIM'D COPPER COMO *14 AWG 6. THERMOPLASTIC JACKET 

2. RUBBER INSULATION COLORED BRAID 7. RUBBER FILLED TAPE 

3. RUBBER FILLER 8. CUTCHED JUTE 

4. 12-7 STR (4 COPPER-3 STEEL) COND 9. ARMOR *12 BWG - 

*16 AWG 33 STRANDS 

5. COLORED THERMOPLASTIC INSULATION (12) 10. IMPREGNATED JUTE 

Figure 3. Main cable construction. 

Splice Boxes. The waterproof splice boxes are 
built to withstand high water pressure and pro- 
vide continuity of the outer cable protection. 
Circular end plates, 6 inches in diameter, con- 
tain standard rubber-washer type glands and 



1. 4-7 STR ALLOY COAT COPPER COND 

*14 AWG 

2. RUBBER INSULATION 

3. COLORED RAYON BRAID(BK,W; BI.GR) 


5. RUBBER FILLED TAPE 

6. CUTCHED JUTE 

7. ARMOR 108 P *14 BWG; 109 P 

*12 BWG 


4. THERMOPLASTIC JACKET 8. IMPREGNATED JUTE 


Figure 4. Lateral cable construction. 


are rigidly connected to each other by steel 
straps. The outer covering is a cylindrical steel 
case. Cable-armor clamps are provided at each 


end (Figure 5) . Although the cases are designed 
to be waterproof in themselves, additional pro- 
tection is provided by waterproofing each indi- 



Figure 5. Straight-through splice case. 

vidual conductor splice with DR tape and by 
filling the case with a low-melting-point potting 
compound. On test, the case alone has with- 
stood a hydrostatic pressure of over 1,000 psi. 

Relays. A conventional type of relay was 
selected for switching between hydrophones, as 
it was believed that the reliability of the glass- 
enclosed vacuum-contact relays used in the 
Block Island installation was not sufficiently 
established by adequate field trials. The unit se- 
lected (Western Electric U.A. Type D163523) 
has four sets of normally open contacts and 
operates on a current of from 6 to 50 milliam- 
peres with a maximum power dissipation of 7 
watts. The relays are enclosed in steel boxes 
tested to withstand a hydrostatic pressure of 
100 psi. 

Attenuating Networks. In order to equalize 
the cable transmission losses from the different 
hydrophones, attenuating networks are pro- 
vided in series with each hydrophone except the 
most remote on each circuit. The pads are in- 
stalled in the terminal boxes on the tripods and 
are connected between the relay and the hydro- 
phone so that they are in the circuit only when 
that hydrophone is in use. The transmission 
loss of the pads is such as to equalize the gen- 
eral transmission level of the signals at the re- 
ceiving end, but it does not correct for differ- 
ences in the transmission-frequency character- 
istic of different lengths of cable. 

T eliminating Networks. The seaward end of 
the cable is terminated in a network which 
matches the cable impedance in order to pre- 
vent serious reflections on the cable and to 


CAPE HENRY INSTALLATION 


175 


make the bridging losses uniform at each point 
where the laterals connect to the main cable. 
Full-coil impedance is simulated by a 325-ohm 
resistance bridged by a 0.04-/xf condenser and 
connected to the circuit beyond the final load- 
ing coil. Use of this termination prevents 
resonances which would otherwise cause large 
variations with frequency in the bridging 
losses, depending on the position of the partic- 
ular hydrophone in use. 

Tnpods. The tripods used in the Cape Henry 
installation are similar to those used at Block 
Island but are constructed of extra heavy 
inch steel pipe. Heavy concrete blocks at the 
bottom of each leg make the total weight of the 
tripod, hydrophone, and terminal box about 700 
pounds. A 600-foot 4-inch manila line, chain- 
weighted at the end, provides a means of hoist- 
ing the tripod without the necessity of drag- 
ging for the lateral cable. A complete hydro- 
phone-and-tripod assembly is shown in Fig- 
ure 1. 

Hydrophones. The Brush C-37 hydrophone, 
which was selected on the basis of the tests 
made at Block Island, is a Rochelle salt crystal 
unit having eight parallel-connected crystal 
elements and a built-in, step-down transformer. 
The crystals are encased in an oil-filled cylin- 
drical rubber container 55 inches long and 21/2 
inches in diameter. Reinforcement inside the 
rubber covering provides for clamping of the 
unit at two points. Figure 6 shows the average 
measured sensitivity of five C-37 hydrophones 



Figure 6. Measured sensitivity of C-37 hydrophone 
and computed “equivalent sensitivity’’ with 60,000 
feet of cable. 


without cable and the computed equivalent sen- 
sitivity“ of this unit when connected to twenty 
3,000-foot lengths of cable with the cable wet 
and dry, loaded and unloaded. 

Listening Amplifier. The amplifier (U. S. 
Navy CDI-50123; uses rectifier CDI-20186) 
used with the Cape Henry installation is a 
production model of a Unit designed in accord- 
ance with requirements indicated by the Block 
Island installation. It permits either headphone 
or loudspeaker listening. The input impedance 
is 200 ohms balanced to ground, and 130 db 
of gain with a maximum power output of 1 
watt at 5 per cent total harmonic distortion is 
provided. Normal frequency response is flat 
within 3 db between 70 and 12,000 c, although 
high-pass filters with cutoff frequencies of 600, 
1,200, and 2,400 c are available. 

Sivitching Circuit. A simplified schematic 
diagram of the hydrophone switching equip- 
ment is shown in Figure 7. This equipment 
automatically switches the individual hydro- 
phones of the cable system into the listening 
circuit allowing the operator to listen succes- 
sively to the various units and spend a selecta- 
ble interval of time on each. Listening intervals 
of 2, 3, 5, 71 / 2 , Of 10 seconds are made available 
on each hydrophone by means of a motor- 
driven cam-switching mechanism. Provisions 
are made both for repeating the automatic 
selection of desired hydrophones and for dis- 
abling the automatic timing and selector cir- 
cuits to permit extended listening on any par- 
ticular hydrophone when desired. Switching 
transients are suppressed automatically by 
placing a short circuit momentarily across the 
listening amplifier output. 

Cable Transmission Characteristics. Values 
of the attenuation and phase shift per nautical 
mile of the main cable with and without load- 
ing, as computed from constants determined 
by measurements on sample lengths, are shown 
in Figure 8. Although submergence of the cable 
in sea water for a period of several months ap- 
pears from these data to have little effect on 

a The equivalent sensitivity is the open-circuit voltage 
in decibels referred to 1 volt appearing across an ideal- 
characteristic impedance terminating network when 
the hydrophone is subjected to a sound pressure of 
1 dyne per sq cm and the hydrophone is connected to 
the terminating network through the length of cable 
with which it is to be operated. 




176 


HARBOR PROTECTION SYSTEMS 


PROGRAM PAIRS 
A B 



Figure 7. Schematic diagram of switching unit. 


RECOMMENDATIONS 


177 


the phase shift, the attenuation per unit of 
length is increased appreciably particularly at 
the higher frequencies. Actual tests on an 8- 
mile length of dry cable just before laying in- 
dicated losses which are in reasonable agree- 




Figure 8. Computed propagation constants of 
cable: (A) nonloaded cable; (B) cable with 6 mh 
per 1,000 yards loading. 


ment with the calculated values. Measurements 
made at this time also indicated that the cross 
talk between the listening pairs varies from 
— 59 db at 1 kc to —49 db at 10 kc. 


Performance 

During preliminary tests, considerable a-c 
hum, originating in a 2,000-yard section of 
cable between the listening post and the beach, 
was present. This section of cable was paral- 
leled for a considerable distance by a 2,300-volt 
3-phase power line and lower voltage secon- 
daries. 


Although the use of the 600-cycle high-pass 
filter was effective as a temporary expedient, it 
was felt desirable to increase the signal-to- 
noise ratio of the incoming signal by installing 
repeater amplifiers where the cable emerges 
from the sea. These amplifiers, housed in a 
weatherproof ventilated metal cabinet, are sup- 
plied with d-c power oyer three spare control 
conductors of the cable from the listening 
post. They have a substantially flat frequency 
response between 100 and 12,000 c, and a gain 
of approximately 40 db. 

In addition to the hum picked up directly 
by the listening conductors, it was found that 
the control conductors in the shore section of 
the cable also picked up hum. As these hum 
currents caused interference with the listening 
circuits, low-pass filters for each control wire 
were installed on the seaward side. 

After these and other lesser difficulties were 
corrected, the system performed satisfactorily. 
No significant interference has been experi- 
enced from background noise of acoustic origin 
other than normal water background noise, and 
consistent detection ranges of over 7,000 yards 
have been attained on surface ships. 

RECOMMENDATIONS 

A number of means of improving or simpli- 
fying the performance of cable-connected 
hydrophone systems have been devised since 
construction of the Cape Henry system. 

Hydrophones. A 5-foot long, permanent-mag- 
net magnetostriction hydrophone, NL-124 
(CQA-51074) offers several advantages over 
the Brush C-37 unit: (1) Its lower impedance 
eliminates the necessity of a line transformer 
and is virtually independent of temperature. 
(2) Its sensitivity is higher. (3) It is capable 
of withstanding the more severe shock pres- 
sures of nearby depth charge or mine explo- 
sions. (4) Its construction makes it easy to as- 
semble. (This unit is capable of operation 
under hydrostatic pressures up to 500 psi.) 
Installed in a vertical position on a tripod, this 
hydrophone discriminates against sound origi- 
nating directly above and when equipped with 
a suitable baffle it may be made essentially uni- 

*’ Calibration of this unit is given in Chapter 6, Volume 
11, Division 6. 


178 


HARBOR PROTECTION SYSTEMS 


directional. This permits discrimination against 
ship sounds within a busy harbor, or against 
surf noise, while at the same time retaining 
normal response in the seaward direction. 

Cable Loading. After the effects of prolonged 
submergence on the leakage conductance of the 
cable were determined, the benefits from load- 
ing were found to be much less than originally 
expected. For this reason it was decided to rec- 
ommend the omission of loading on any future 
cable-connected hydrophone systems using this 
type cable. It is expected that the use of the 
nonloaded cable will result in (1) greater sim- 
plicity and lower cost, (2) improvement of cir- 
cuit balance with consequent reduction of noise 
pickup, (3) reduction of the effect of cable 
leakage upon attenuation, (4) improved im- 
pedance match between hydrophones and cable, 

(5) lower thermal noise from the cable, and 

(6) greater reliability. In addition, the use of 
nonloaded cable enables the future transmis- 
sion of supersonic signals in the event this 
should appear desirable inasmuch as the h-f 
cutoff introduced by loading would make such 
transmission impossible. 

Hydrophone Matching Networks. The opti- 
mum condition of matching between the hydro- 
phone and cable impedances occurs when the 
hydrophone impedance is the conjugate of that 
of the cable. It is possible, therefore, to produce 
some improvement in the transmission of the 
higher frequencies by providing a transformer 
to step up the hydrophone impedance together 
with a shunting condenser to nullify the posi- 
tive reactance of the hydrophone at a selected 
frequency. However, the slight increase in 
transmission efficiency obtainable by the use of 
such matching networks does not warrant their 
use on the lengths of cable usually necessary 
for harbor protection systems, as the acoustic 
background noise is generally well above the 
thermal noise on such lengths of cable. On un- 
usually long cables, where thermal noise ap- 
proaches the water noise level, it may be of 
advantage to provide matching networks. 

Splicing Procedure. On the Block Island and 
Cape Henry cables, straight color-to-color 
splices were used between the listening con- 
ductors in the various sections. By making 
capacity-unbalance tests and splicing the con- 
ductors, regardless of color, so as to equalize 


the capacity to ground, the circuit balance can 
be improved and the susceptibility to inductive 
interference reduced. 

Equalization of Signal Levels. With fixed at- 
tenuation pads installed at the tripods to adjust 
the levels of the signals from the various hydro- 
phones on the Cape Henry system, no provision 
is made for compensating for possible changes 
of hydrophone sensitivity with time. An alter- 
native method of equalizing the levels is recom- 
mended in which the attenuation pads would be 
installed at the listening station and selected 
by additional arcs on the rotary selector switch 
or by means of relays controlled by the existing 
selector mechanism. This type of equalization 
could be employed at the amplifier output and 
the pads made variable by means of screw- 
driver adjustments. 

Timing Circuit. In order to provide variable 
listening intervals without the use of a series 
of motor-driven cams, an electronic timing cir- 
cuit employing a gas-filled, cold cathode tube 
is suggested. Such a circuit, in addition to 
eliminating the cumbersome motor and cam 
mechanism, would provide for continuously 
variable intervals instead of the predetermined 
intervals necessitated by the cam arrangement. 

Relay Sivitching Circuit. Another arrange- 
ment, consisting of a relay chain circuit instead 
of a rotary selector switch, has been successfully 
tested. Similar to the counting circuits em- 
ployed in standard telephone service, it provides 
automatic and manual selection of individual 
hydrophones by means of keys. 


The Anchored Radio Sono Buoy 
[ARSB] 

The anchored radio sono buoy [ARSB] is a 
device ivhich is anchored in a harbor to pick 
up enemy submarine sounds and transmit them 
by radio to shore receiving stations. The final 
model [JM-1] consists of tivo buoys connected 
by cables. One buoy contains a transmitter 
2 vith maximum deviation to 75 kc, a small non- 
directional hydrophone suspended beloiv on a 
cable, and a half-ivave antenna. The other buoy 
houses a removable 20-day dry-cell battery 
po 2 ver supply and the anchor. Important design 


DEVELOPMENT 


179 


features of the equipment are the use of pre- 
emphasis of high frequencies, high audio gain, 
acoustic padding inside the transmitter buoy 
to reduce microphonics, heater-type tubes, and 
a quarter-wave ship antenna with ground plane. 
The original ARSB, known as the JM buoy, 
was developed by the NRL for the Bureau of 
Ships. Later NDRC contidbutioyis were in co- 
operation with the Brush Developmeyit Com- 
pany in the selection of the hydrophone, and 
geyierally small but important suggestions for 
improvement of the early production models, 
the development of methods of anchormg a 7 id 
managing the cables to eliminate 7 ioise arid 
tangling, and assistance in personnel training. 


sounds, picked up by the hydrophone, are am- 
plified and impressed by means of FM on a 
radio carrier wave. Trained listeners at a near- 
by shore listening post can then maintain a 
continuous watch on ship sounds in the area. 

The final model, JM-1, operated satisfactorily 
at an 8-mile radio range and equaled the cable- 
connected system in j acoustic performance 
under favorable weather conditions. The buoy 
itself could not withstand severe storms. Sug- 
gestions for further research and development 
propose incorporation of all components in one 
streamlined buoy and inclusion of a device to 
enable alternative directional and nondirect- 
tional use of the hydrophone. 



Figure 9. The JM-1 anchored radio sono buoy. 


INTRODUCTION 

The ARSB is reserved for use in deep water 
where laying of cables is not feasible, and for 
auxiliary or emergency use at advanced bases 
where the expenditure of time, material, and 
effort needed for a cable-connected hydrophone 
system would not be justified. Underwater 


DEVELOPMENT 

* * The JM Buoy 

The JM buoy assembly, shown in Figure 10, 
consisted of two buoys, one a transmitter buoy 
from which a hydrophone was suspended, and 
a separate anchor buoy to prevent cable foul- 
ing. The electric equipment consisted mainly of 
a dry-cell power supply, a medium-gain audio 
amplifier, an FM carrier wave system, and a 
half-wave antenna. 

A large number of field tests were carried 
out in cooperation with the Brush Development 
Company to determine the most suitable hydro- 
phone available and the optimum coupling of 
the hydrophone to the amplifier. Under different 
weather conditions, different types of hydro- 
phones gave the best performance. However, 
the small C-23 Brush hydrophone was recom- 
mended as giving the most satisfactory all- 
round performance. 

It might be presumed that a long hydro- 
phone, one with directional characteristics to 
discriminate against surface sounds, would be 
especially suited to this device. However, when 
suspended in the usual way from an anchored 
buoy, the hydrophone may be deflected from its 
vertical position by a water current of any ap- 
preciable magnitude. A long hydrophone 
thereby defeats its own purpose. 

The automatic volume control [A VC] in- 
corporated in the JM audio amplifier was found 
to reduce the sensitivity of the buoy by about 


180 


HARBOR PROTECTION SYSTEMS 


10 db for moderate signal levels. It was accord- 
ingly disconnected in all JM transmitters. 

Further tests indicated that the buoy was as 
efficient as could be expected with the filament- 



type tubes available. However, it was believed 
that considerably improved performance might 
be obtained with other tubes. 


Experimental Transmitter 

As a result of the experience gained with the 
JM buoy a new experimental transmitter was 
designed and constructed to fit into the JM 
transmitter housing. 

Low microphonics, high audio gain, fre- 
quency multiplication, greater frequency devia- 
tion, and increased radio signal intensity were 
featured in the new transmitter. Increased 
audio and radio range as well as improved fre- 
quency stability were all clearly evident, but 
the frequent (every 3 days) servicing, neces- 
sitated by the limited battery space of the JM 
design, caused its abandonment. 

The JM-1 Model 

The JM-1 model incorporates a number of 
the features found desirable in the JM design. 
Among the more important design features are 


the heater-type tubes, high audio gain, low 
microphonics, and pre-emphasis of high fre- 
quencies. 

Figure 9 shows how this buoy appears in the 
water, and Figure 11 how the components are 
related. Since, in this model, the battery pack 
is housed in the anchor buoy, a battery cable 
as well as a tie cable is needed between the two 
buoys. The added battery space affords added 
battery capacity and thus extends service 
periods to 20 days or more. 



Figure 11. Diagram of JM-1 buoy in the water. 


The use of pre-emphasis of high frequencies 
in the transmitter had been recommended as a 
result of accumulated experience in underwater 
listening. It was known that higher frequencies 
are very useful in detection and that the energy 
content of underwater noise decreases fairly 
rapidly with increase in frequency. However, 
self-noise in a receiver increases with fre- 
quency. Above a certain frequency, with uni- 
form amplification in both the receiver and the 
transmitter, receiver noise masks the reception 
of the desired underwater sounds. This diffi- 
culty can be overcome by emphasis of the high 
frequencies in the buoy transmitter and the 
de-emphasis of these frequencies in the shore 


SUGGESTIONS 


181 


receiver. This achieves a highly desirable in- 
crease in signal-to-noise ratio in the critical 
h-f region at the cost of a readily tolerable de- 
crease in this ratio at lower frequencies. 

After completion of laboratory tests and 
measurements, the JM-1 model was anchored 
in 185 feet of water at a distance of about 8 
miles from the receiver. For several days the 
buoy operated satisfactorily and many ships 
were heard. However, during a severe storm 
one night the buoy disappeared and no trace of 
it was found. 

On the basis of the laboratory and field tests 
completed, researchers recommended develop- 
ment of a new buoy with increased pre-empha- 
sis to conform to the standard FM pre-empha- 
sis characteristic, improved frequency stability, 
increased maximum deviation to 75 kc, and in- 
creased mechanical strength of the buoy. 

14.7.3 JM-1 Production Buoy 

The first four of the first lot of JM-1 produc- 
tion buoys ordered by the Navy were subjected 
to extensive laboratory and field tests. At vari- 
ous times they were anchored within audio 
range of a considerable amount of ship traffic. 
They appeared to be quite seaworthy, at least 
as far as summer weather and water conditions 
in Long Island Sound could demonstrate. How- 
ever, underwater sound pickup was not ideal. 
Water noise generated by the transmitter buoy 
and the battery float tended to mask the desired 
signal to a degree dependent on the strength of 
the water current and the state of the sea. 

Mechanical problems encountered in connec- 
tion with the above tests included parting of 
the tie cable, leaky buoys, crossing and tangling 
of cables, and general unwieldiness of the 
equipment. Parting of the tie cable, originally 
believed to have been caused by ships overrun- 
ning the cable, was finally attributed to inter- 
crossing of the tie and anchor cables, with a re- 
sulting sawing action. A solution was found in 
the use of a chain yoke fastened to opposite 
sides of the battery container, with a swivel 
for connecting to the anchor cable. With this 
arrangement, any crossing was cleared by the 
turning action exerted as the tie cable pulled 


against one side of the yoke. Crossing of the 
hydrophone cable with the tie and battery 
cables was eliminated by the use of floats on 
the latter. In order to detect leaky buoys and 
battery containers before planting the buoys, 
air pressure of several pounds pumped into 
each unit was released. The rate of drop in 
pressure was observed on a pressure gauge. 

Training Records 

In connection with the above activities a 
series of phonograph records was prepared for 
training operators of ARSB equipment. The 
records provided samples of noises from two 
types of submarines and from a number of sur- 
face craft. The series was made complete 
enough so that by careful study and practice 
the operator might acquire the ability to rec- 
ognize most of the sounds usually encountered. 

SUGGESTIONS 

To overcome some of the difficulties en- 
countered in the JM-1 buoy, it has been sug- 
gested that the design be altered to utilize a 
simple streamline buoy having the hydrophone 
mounted just above the anchor. The single 
buoy would house both the transmitter and the 
battery pack and thus eliminate some of the 
unwieldiness of the JM-1 buoy. If streamline, 
it would generate less water noise and, by de- 
creasing the relative effect of wind on its move- 
ment, reduce the possibility of cable fouling. 
By placing the hydrophone near the anchor the 
disturbing effects of surface noise would be 
minimized. Furthermore, a fixed position would 
allow the use of a directional hydrophone. 

The success of the directional radio sono 
huoy [DRSB], discussed in Chapter 9, makes 
it appear logical to apply similar directional 
principles to the ARSB. In this way the maxi- 
mum range of detection might be increased be- 
cause of the directivity of the hydrophone. In 
addition the indication of the actual direction 
of the sound would be valuable. It would seem 
wise, at the possible expense of added compli- 
cation, to use both directional and nondirec- 
tional listening. 



Chapter 15 


SUBMARINE COMMUNICATION SYSTEMS 


T his chapter deals with the studies and 
tests of two types of systems, one for sub- 
marine internal communications and the other 
for communications between submerged sub- 
marines and between submarines and surface 
craft. Where small, closely adjoining compart- 
ments or the necessity for underwater opera- 
tions must be considered, a salient problem is 
intelligibility. 

The crowded quarters and high noise levels 
of submarines impose difficult requirements for 
intelligibility on submarine internal communi- 
cations. Adaptation of the 7-MC, or talkback 
system, involved eliminating extraneous noise 
picked up by the large microphones in the con- 
ning tower, control room, and on the bridge, 
and overcoming feedback between the acous- 
tically coupled compartments. The main prob- 
lem connected with improving the 1-MC, or 
general announcing system, arose at the speak- 
ers in the engine rooms where it was difficult 
for the operators to hear messages over the 
din of the motors. 

The underwater telephony system described 
provides communications between ships rather 
than within a single ship. Unlike surface 
craft, submarines are limited, in this problem, 
to underwater methods which are subject to the 
same oceanographic factors encountered in lis- 
tening. To improve intelligibility and maximum 
range, several methods of modulation were in- 
vestigated. It is interesting to note that the use 
of FM, rather than increasing, actually reduced 
intelligibility. A single side-band, suppressed- 
carrier system was found to be most satis- 
factory. 

Submarine Internal Communications 
System 

Submarine internal communications systems, 
as described here, are improvements on the 
7-MC (talkback) and 1-MC (general announc- 
ing) systems adapted for submarines from 


similar surface ship installations. The project 
described ivas primarily a set of studies and 
tests aimed at overcoming some of the innate 
difficulties encountered in setting up a system 
of internal communications on a small vessel. 
Change from large sensitive microphones to 
close-talking ones in the conning tower, control 
room, and on the bridge answered salient noise 
problems of the 7-MC system. Increased 
speaker level with attenuation at the operator's 
ears remedied outstanding shortcomings of the 
1-MC system. Observations and tests follotved 
by improvements and further recommenda- 
tions were carried out by the New London 
Laboratory. Trial installation of equipment 
based on the recommendations shoiued an im- 
provement in intelligibility. 

INTRODUCTION 

The complexity of new equipment developed 
to aid in the operation of submarines imposed 
severe requirements on the performance of 
internal communications systems. Better in- 
telligibility and greater coverage were held 
essential. In an effort to provide a solution, 
the 1-MC and 7-MC systems had been adapted 
from similar surface ship installations. How- 
ever, because of the markedly different con- 
struction and the highly specialized communi- 
cation requirements of submarines, serious dif- 
ficulties developed in the operation of these 
systems. Modifications of the systems were 
recommended and carried out in a trial instal- 
lation on a submarine. Tests made of these 
modifications showed that sentence intelligi- 
bility under most conditions of ship operation 
was increased to excellent as contrasted with 
the fair-to-poor intelligibility existing pre- 
viously. 

As furnished, the 1-MC system provides 
loudspeakers in each compartment and micro- 
phones in the conning tower and control room. 
The 7-MC system employs loudspeakers and 


182 




MICROPHONE PERFORMANCE TESTS 


183 


transducers and serves only the bridge, con- 
ning tower, control room, and the forward and 
after torpedo rooms, with provision for talk- 
ing from any of these stations to all of the 
others. 

EARLY TESTS 
The 1-MC System 

Early observations and articulation tests' ^ 
indicated that the 1-MC system in its original 
form gave generally satisfactory performance, 
but fell short of requirements in the engine 
rooms with the diesels operating, where the 
ambient noise level at times approached the 
threshold of feeling. Three 25-watt speakers 
are provided in each engine room and adequate 
power is available in the 1-MC system to over- 
ride the ambient noise sufficiently from a phys- 
ical standpoint. Physiologically, however, a 
signal-to-noise ratio which would be adequate 
for intelligibility at lower levels does not give 
good results when the ambient noise is near the 
threshold of feeling. Under these conditions 
the ear’s response is relatively insensitive to 
amplitude differences. To improve intelligi- 
bility, both the signal and the noise must be 
made less intense at the ear. 

The 7-MC System 

Articulation tests on several typical 7-MC 
system installations indicated that the sentence 
intelligibility over this system was no better 
than fair to poor for most conditions of ship 
operation. The 7-MC system, susceptible to 
noises and feedback, did not provide satisfac- 
tory communication. Major difficulties resulted 
from excessive wind, sea, and exhaust noise 
from the speaker-microphones on the bridge, 
noise pickup during certain conditions of ship 
operation, and acoustic feedback resulting from 
close acoustic coupling between the speakers 
and speaker-microphones. 

Wind Noise. Laboratory tests with a bridge 
speaker and a fan indicated the major source 
of wind noise to be pulsations generated at 
two holes opening into the space behind the 
diaphragm. Since the holes are necessary to 
allow flooding of this space when the ship sub- 


merges, a collar was designed which prevented 
wind from blowing across them but at the same 
time allowed water to flood the unit normally. 
In addition, a fine mesh screen across the front 
face of the diaphragm reduced the wind veloc- 
ity. These measures reduced the wind noise by 
10 to 12 db. 

Feedback. Relocation of the speakers in the 
conning tower and control room represented an 
attempt to reduce acoustic feedback. Trials 
indicated that it was possible to reduce the 
feedback appreciably by this means, but the 
resultant speaker locations were not generally 
satisfactory for adequate coverage of the com- 
partments. No rule could be made for best 
speaker location even from the standpoint of 
feedback alone, presumably because of minor 
individual ship differences in the location of 
other pieces of interfering equipment. 

Recommendations 

These tests and observations led to the con- 
clusion that problems involving feedback, lo- 
cation of speakers, reduction of noise pickup, 
and optimum adjustment of speaker levels 
could best be solved in the 7-MC system by 
divorcing the dual function of the speakers 
(then used as both speakers and microphones) 
and providing close-talking microphones in the 
conning tower and control room. 


MICROPHONE PERFORMANCE 
TESTS 

Close-Talking Microphones 

Articulation tests were made under operat- 
ing conditions to compare the performance of 
several close-talking microphones in the 7-MC 
system with the performance of the original 
speaker-microphone. These tests supported the 
conclusion that the close-talking units yielded 
considerably improved intelligibility. They also 
confirmed previous qualitative tests made in 
the laboratory which had indicated that a wide- 
range, electromagnetic headset receiver used 
as a microphone gave excellent speech intelli- 
gibility. The frequency-response characteristic 
of this unit is shown in Figure 1. 


•o 


184 


SUBMARINE COMMUNICATION SYSTEMS 


Speaker-Microphones 

The 7--MC speakers operated satisfactorily 
as microphones only under the quieter condi- 
tions of ship operation and then only after the 
units in the control room and conning tower 



Figure 1. Frequency response characteristic of 
Permoflux ANB-H-IA receiver used as a micro- 
phone. 


had been painstakingly placed for maximum 
reduction of feedback rather than in the opti- 
mum locations for coverage of the compart- 
ments. 

Recom mendations 

* 

At the conclusion of these tests it was sug- 
gested that an improved 1-MC system provide 
an automatic means of lowering the engine- 
room speaker levels when diesels are not run- 
ning and provide ear plugs for use of the 
engine-room crew when diesels are running. 
It was also recommended that a modified 7-MC 
system substitute multiple close-talking micro- 
phones in the conning tower and control room 
for the speakers then being used as micro- 
phones. To provide for better sound distribu- 
tions in the conning tower and control room, 
installation of two speakers each in these com- 
partments was suggested. In compartments 
where the ambient levels were subject to large 
variations, an automatic means of boosting the 
level from loudspeakers was recommended. In- 
sulation of all loudspeakers from the hull or 
bulkheads by means of vibration - insulating 
mounts could prevent feedback due to mechani- 
cal vibration. The mounts should be stiff 
enough for the mechanical-resonance frequency 
of the speakers to be low compared to the peak- 
response frequency of the electroacoustic sys- 


tem. Minor innovations would include a hand- 
set for auxiliary use on the bridge, circuits 
which would eliminate the necessity for using 
press-to-talk switching at the key station, sepa- 
rate circuits for each external station to re- 
duce the possibility of flooding-out failure of 
communications, and bridge pickup units de- 
signed to reduce or eliminate wind noise. 

Although no original work was undertaken 
on the determination of signal-to-noise ratios 
or pass-band requirements for optimum intel- 
ligibility in the presence of noise, the works 
of H. Fletcher and N. R. French^*^ on those 
subjects were freely consulted. However, some 
studies were made of the characteristics of the 
noise in various compartments of several sub- 
marines. These studies revealed that maximum 
noise energy was below 1,000 c in most in- 
stances. Higher frequency components were 
found, however, in sounds from escaping air, 
certain types of low-pressure blowers, diesel 
engines at high speeds, etc. Thus all frequencies 
had to be considered in modifying the system. 

154 the modified system 

Modifications in accordance with the above 
recommendations were incorporated in a typi- 
cal 7-MC installation on a new submarine. Cir- 
cuit changes were confined almost wholly to 
changes in the ship’s wiring and to the provi- 
sion of new switching arrangements.^ Five 
close-talking microphones were installed in the 
conning tower, two in the control room. The 
microphones were adapted from wide-range, 
electromagnetic headset receivers installed 
in 1-MC microphone cases with press-to-talk 
switches. The locations of these microphones 
in the conning tower are shown in Figure 2. 
Control-room microphones were located at the 
diving officer’s station and near the radar sta- 
tion. Provision was made for raising and 
lowering the loudspeaker levels by means of 
a pressure-actuated switch in the low-pressure 
blower lines in the control room and by peri- 
scope limit-switches attached to the diesel ex- 
haust valves in the engine rooms. Tests, ob- 
servations, and discussions with Navy repre- 
sentatives indicated the desirability of three 
additional features in the modified system. 
Because of the high noise level in the torpedo 



THE MODIFIED SYSTEM 


185 


rooms during firing, installation of close- 
talking microphones there in addition to those 
in the conning tower and control room would 
simplify communications during an attack; a 
close-talking microphone on a short gooseneck 
to be plugged in on the bridge could be used 


letter articulation. Each figure reported in the 
table represents the average number of cor- 
lectly heard sounds out of a total of several 
hundred spoken words. It is generally con- 
sidered that letter articulation below 60 per 
cent is indicative of poor sentence intelligibil- 



in place of the handset or speaker microphone 
when desired ; and provision for switching the 
forward and after torpedo-room speakers on 
and off from the conning tower would simplify 
operations and reduce noise. 

Comparison of Articulation Percentages 

A comparison of all the articulation per- 
centages from the early tests on unmodified 
7-MC systems using speaker microphones with 
those obtained using the wide-range, close- 
talking microphones is given in Table 1. These 
articulation tests were conducted at sea under 
actual operating conditions. In each case three 
of the submarine’s officers were employed as 
talkers from every station under all conditions 
of operation. Each spoke 50 words from a 
standardized articulation word list. Two to 
four laboratory representatives at different re- 
ceiving stations recorded the words heard over 
the system. The results were checked for the 
percentage of correctly heard sounds— so-called 


ity, whereas above 80 per cent gives excellent 
intelligibility. 


Table 1. Comparison of letter articulation per- 
centages obtained in tests on submarines having 
modified and unmodified 7-MC systems.* 


Operating conditions 

Un- 

modi- 

fied 

7-MC 

system 

Un- 

modi- 

fied 

7-MC 

system 

Modi- 

fied 

7-MC 

(close- 

talking 

micro- 

phone) 

Modi- 

fied 

7-MC 

(close- 

talking 

micro- 

phone) 

Day attack (sub- 

58 

59 

92 

90 

merged) 




Night attack (sur- 

47 

68 

94 

90 

faced) 




Blowing up (surfaced) 

26 

No data 

85 

87 


*'The reader is cautioned not to place too much emphasis on the 
exact differences in percentage between the modified and unmodi- 
fied scores in the above comparison, as unavoidable differences in 
the tests may be reflected in the percentages obtained. However, 
the^ broad aspect of the comparison, contrasting fair-to-poor in- 
telligibility on the earlier boats with excellent intelligibility on the 
later ones having modified systems, is valid. 


186 


SUBMARINE COMMUNICATION SYSTEMS 


15.5 CONCLUSIONS 


This development program was undertaken 
to provide modifications which could be applied 
to existing equipment as quickly as possible in 
wartime. Although the modified 1-MC and 
7-MC systems yielded improved results over the 
first systems, they fell short of optimum per- 
formance in a number of ways. 

Incorporation of a close-talking microphone 
permanently installed on the bridge, improved 
switching relays for both systems, and a con- 
veniently controllable microphone switch for 
the helmsman would represent important im- 
provements in the modified system. 

A permanently installed, close-talking micro- 
phone on the bridge is necessary to insure 
maximum reduction of noise from wind, sea, 
and diesel exhausts. No unit which withstands 
pressure to the necessary degree and which 
has the required sensitivity, fidelity, and rug- 
gedness was available. 

It is important for communications systems 
for this type of service to be as completely re- 
liable as possible under all conditions of ship 
operation ; thus they should require a minimum 
of field servicing. The circuit switching in both 
the 1-MC and 7-MC systems employs a consid- 
erable number of relays which are not wholly 
proof against the shock, vibration, and high- 
humidity conditions likely to be encountered. 
The relays should either be eliminated or re- 
duced in number, or a type of relay should be 
developed which will stand up better under ad- 
verse operating conditions. 

The helmsman is required to acknowledge 
commands, and his means of communication 
with other compartments and the bridge is 
limited to the 7-MC system. Because of the 
type of operation provided by this system he 
must use one hand for press-to-talk switching. 
This is not always feasible, particularly when 
the ship is maneuvering in close quarters. 
Provision should be made, possibly by means 
of a foot switch, to permit the helmsman to 
use the 7-MC system without hand switching. 
It is imperative, however, if the present gen- 
eral type of operation is retained, that adequate 
protection be provided against inadvertent op- 
eration of the helmsman’s switch. 



Figure 3. Prototype model transmitting equipment 
for underwater telephony system. 


Underwater Telephony System 

This experimental under IV at er telephony sys- 
tem ivas designed by CUDWR-NLL to provide 
for voice communication between submerged 
submarines or between a submerged submarine 
and a surface ship. The system utilizes subma- 
rine supersonic equipment [WCA-2] plus voice- 
frequency transmitting and receiving circuits 
with their associated filters, a carrier wave 
generator, and a potver amplifier capable of 
delivering approximately 100 ivatts of undis- 
torded potver to the transducers. The system 
proposes a single side-band suppressed-carrier 
type of transmission in which the signal tvas 
found to be less subject to distortion than the 
signal in an FM system for underwater trans- 
mission. A carrier frequency of 26 kc is used, 
a frequency at tvhich the transmitting and re- 
ceiving transducers are highly directional. A 
system operating at 8 kc tvas also investigated. 
Such a system can provide greater range but 
less directionality with the same transducers 
used in the high-frequency system. In tests of 


O 


INVESTIGATION OF FREQUENCY-MODULATED TRANSMISSION 


187 


experimental 26-kc models, consisterit ranges of 
10,000 yards to 15,000 yards were obtained in 
deep or shallow tvater. It ivas found that exces- 
sive reverberatio7i was less troublesome with 
this type of communication than ivith echo- 
ranging code communicatio 7 is. 


INTRODUCTION 

Certain types of naval operations require 
communication between submerged submarines 
and other submarines or surface ships in the 
same vicinity. Code signaling with standard 
echo-ranging equipment meets this need in 
some measure but as this method is slow and 
not always reliable in areas where underwater 
reverberation is high, communication by means 
of voice-modulated supersonic waves was pro- 
posed. Experimental work was undertaken to 
evaluate this type of transmission and, if pos- 
sible, to develop a suitable transmitting and 
receiving equipment. 

First consideration of the factors affecting 
underwater sound transmission suggested that 
frequency modulation rather than amplitude 
modulation would yield the more satisfactory 
performance. Thorough field tests of equipment 
employing frequency modulation failed to show 
the expected results, however, and later tests 
of amplitude-modulated equipment indicated 
greatly superior performance. 

This experimental work led to the develop- 
ment of an experimental amplitude-modulated 
underwater telephony system employing single 
side-band, suppressed-carrier features and ca- 
pable of providing good speech intelligibility 
at maximum ranges of 10,000 to 15,000 yards. 
Operating on a carrier frequency of 26 kc, it 
utilizes existing supersonic gear, with slight 
modifications. Two prototype models of the 
equipment installed on submarines at Pearl 
Harbor performed satisfactorily. 

PRELIMINARY SURVEY OF THE 
PROBLEM 

As a result of previous experience it is 
known that any underwater communication 
system utilizing supersonic waves is influenced 
by a number of factors. 


1. Transmission loss suffered by a single-fre- 
quency supersonic tone in traveling several 
thousand yards through the water may vary 
as much as 20 db in an interval of a few sec- 
onds. 

2. Multiple transmission paths often cause 
excessive reverberation which frequently 
masks the intelligibihty of supersonic code 
signals. 

3. Background noise, always present in the 
water, and the noise produced by motion of the 
receiving vessel limit the range of useful trans- 
mission. 

In power-line carrier-current transmission 
of voice-modulated signals, such factors as 
transmission loss variations, interfering noise, 
and impedance irregularities are similar to 
the limitations encountered in underwater 
sound transmission. In frequency-modulated 
systems with power-line carrier transmission, 
the limiting action of the receiver and charac- 
teristic behavior of the discriminator overcome 
the troublesome effects of transmission loss 
variations and improve the effective signal-to- 
noise ratio. It was expected, therefore, that 
similar equipment might be equally successful 
in the case of underwater voice communica- 
tion. 

15.8 INVESTIGATION OF FREQUENCY- 
MODULATED TRANSMISSION 

Power-Line Carrier System 

In assembling gear for investigating the 
feasibility of frequency-modulated supersonic 
voice communication, advantage was taken of 
equipment already available. The General 
Electric FM power-line carrier equipment, 
using an intermediate frequency of 175 kc 
which could readily be retuned to the inter- 
mediate frequency used by the echo-ranging 
receiver, was well adapted for use with the 
power amplifiers, transducer coupling circuits 
and transducer available in the WCA-2 echo- 
ranging gear already on submarines. The 
equipment needed for the tests, therefore, in 
addition to the standard echo-ranging gear, 
included only a special FM receiver and the 
low-level portion of the power-line transmitter. 

A one-way transmission system utilizing 


188 


SUBMARINE COMMUNICATION SYSTEMS 


these components and capable of delivering an 
electric input of approximately 5 watts to the 
JK transducer was installed on two surface 
ships for sea tests.® In this equipment the re- 
ceiver had a sensitivity of approximately 50 /xv 
at its input terminals. Although transmission 
was reasonably successful with ranges up to 
5,000 yards using a carrier frequency of 27 kc 
and up to 1,500 yards using 70 kc, reception 
was characterized by background noise. It was 
thought that this was possibly due to inade- 
quate range in the limiter. 

Improved System 

Believing that reception might be improved 
with a more sensitive receiving circuit, new 
equipment was designed incorporating receiv- 
ers of l-/xv sensitivity and providing for two- 
way transmission with a power input to the 
transducer of approximately 100 watts. Sea 


It was noted that the noise increased when- 
ever the signal was modulated. From this evi- 
dence the conclusion was drawn that it resulted 
from the distortion of the transmitted signal 
in the medium. For verification, a high-speed 
moving-film oscillograph was employed to make 
records of the output of the receiving trans- 
ducer. Figure 4 shows typical oscillographic 
records, made with sinusoidal signals of con- 
stant frequency and amplitude impressed at 
the voice input terminal of the transmitting 
circuit. Although it was certain that the elec- 
tric wave impressed upon the transmitting 
transducer contained negligible amplitude mod- 
ulation, it is clear from the oscillograph traces 
that during transmission through the water the 
signal suffered a considerable amount of am- 
plitude modulation by the time the wave 
reached the receiving vessel. The rate of oc- 
currence (750 per second) of the sharp spurs 








0.1 SEC 

Figure 4. Oscillograms of frequency-modulated supersonic waves after transmission through water. Original 
modulation 750 c at constant amplitude, carrier frequency 26 kc, receiver 1,050 yards from transmitter. 


tests of this equipment failed to show the ex- 
pected results. After the entire gear was 
checked and readjusted in the laboratory, fur- 
ther sea tests were made but the equipment 
still failed to perform satisfactorily. The over- 
all performance of the system was character- 
ized by excessive noise and by extremely vari- 
able signal levels. 


defining the envelope of the amplitude modula- 
tion of the received signal corresponds with 
the frequency of the signal used for frequency 
modulation of the original carrier. The per 
cent of amplitude modulation was extremely 
variable, sometimes changing from nearly zero 
per cent to 50 or 60 per cent in the space of 
a few milliseconds. 


INVESTIGATION OF AMPLITUDE-MODULATED TRANSMISSION 


189 


Reasons for Failure of FM System 

Examination of the factors involved dis- 
closed the reasons for failure of this type of 
signal to yield the expected results. Because 
of the nonexistence of ideal medium, every 
wave suffers some distortion during transmis- 
sion. Frequency-modulated signals are no ex- 
ception to this rule. Some of the characteristics 
of an FM wave, particularly its constant ampli- 
tude, require the preservation of certain phase 
relations among its several components. These 
phase relations are disturbed during transmis- 
sion by interference between components re- 
ceived over multiple transmission paths. In the 
case of a high-frequency radio carrier modu- 
lated by an audio-frequency signal, the time 
intervals between arrivals over several paths 
are so short as Jto be entirely negligible com- 
pared to the phase differences in the transmit- 
ted wave. On the other hand, the much slower 
speed of propagation of underwater sound 
causes the time intervals between arrivals over 
multiple paths to be many times greater than 
those encountered in radio transmissions. In 
this instance, the disturbance of phase rela- 
tions in the wave becomes so great that it com- 
pletely destroys the essential characteristics of 
the signal. One consequence of this is the ap- 
pearance of amplitude modulation. When im- 
pressed upon an FM receiver, these distortions 
produce prohibitive amounts of spurious audio- 
frequency components as well as variations in 
the level of the recovered signal. 

Unsuccessful attempts to transmit television 
signals via FM waves confirm this deduction 
of the cause of the observed signal impairment. 
Here the intervals between arrival times over 
multiple paths are the same as in the case of 
radio-frequency transmission of audio signals. 
However, in the case of television, the frequen- 
cies of the components of the original signal 
are so high that the phase relations between 
them are defined by time intervals short enough 
to be comparable with those between arrival 
times. 

The tests indicated that frequency-modulated 
supersonic transmission is not suitable for un- 
derwater voice communication. In view of the 
care with which the circuits were constructed 
and the accuracy with which their performance 


characteristics were known, the results of these 
tests were regarded as conclusive. 

15.9 INVESTIGATION OF AMPLITUDE- 
MODULATED TRANSMISSION 

In designing the circuits used for the final 
tests of frequency-modulated transmission, pro- 
vision was made for amplitude-modulated sig- 
nals to be transmitted and received. This per- 
mitted observation of the comparative perform- 
ance of the two systems. First indications were 
that the performance with amplitude-modu- 
lated signals was greatly superior to that ob- 
tained with frequency modulation. Thus it was 
decided to investigate thoroughly amplitude- 
modulated underwater telephony. 

Theoretical Considerations 

In conventional amplitude-modulated sys- 
tems, the transmitted signal contains a com- 
ponent of carrier frequency called the unmodu- 
lated carrier, together with two groups of side 
band components. Because, in reception, de- 
modulation involves use of the unmodulated 
carrier, satisfactory performance can be ob- 
tained only so long as the carrier component 
is present in the received signal. In under- 
water sound transmission, however, a single- 
frequency tone suffers considerable variation 
in amplitude; at times the summation of 
components reaching the receiving point by 
different water paths may cause complete can- 
cellation. Should this happen to the carrier 
component of a conventional amplitude-modu- 
lated signal there would be a concurrent failure 
to recover the original audio signal. 

Consideration of these difficulties suggested 
employing a specialized type of amplitude- 
modulated system in which the carrier fre- 
quency is deliberately suppressed at the trans- 
mitter and reintroduced at constant amplitude 
at the receiver. Previous experience has shown 
that, in such a suppressed-carrier system, cer- 
tain advantages are gained by the use of a 
single side band rather than the upper and 
lower side bands normally developed by the 
transmitting modulator. With a single side 
band, the total permissible power into the pro- 
jector may be restricted to a narrower band 


190 


SUBMARINE COMMUNICATION SYSTEMS 


and the acceptance band of the receiver may 
be correspondingly narrower with resulting 
reduction of background noise components. In 
addition, the likelihood of undesirable inter- 
ference between components, produced by in- 
termodulation between the carrier and the cor- 
responding components of the side bands, is 
avoided. In using a suppressed-carrier system 
for underwater telephony between vessels, one 
inherent difficulty is the relative motion which 
usually exists between the transmitting and 
receiving transducers. This makes it necessary 
to compensate for doppler shift by displacing 
the carrier impressed at the receiver to that 
frequency which the transmitter carrier would 
exhibit if it traveled through the medium. 

Suppressed-Carrier System 

Equipment. To test the performance of single 
side-band suppressed-carrier transmission, a 
transmitting system was designed consisting 
of a carrier oscillator generating a frequency 
of 26 kc, a speech input amplifier, a single 
side-band filter, and a high-quality power am- 
plifier capable of delivering approximately 100 
watts of undistorted power to the JK or QB 
transducer. For receiving, the WCA-2 echo- 
ranging amplifier^ was used without change 
except for readjustment of the frequency of 
the beat-frequency oscillator. 

Tests. Tests of this system on moving ves- 
sels were uniformly satisfactory. Ranges of 
10,000 yards or more were consistently ob- 
tained in Long Island Sound. At ranges of 
more than 12,000 yards, the background noise 
tended to become excessive, but contact was 
generally not lost completely until a range of 
about 15,000 yards was attained. In deep- 
water tests, communication was possible at 
ranges of 6,000 and 8,000 yards in the pres- 
ence of thermal gradients for which the limit- 
ing ray diagrams showed ranges to the shadow 
zone of only 1,000 and 1,500 yards respectively.® 

The signal levels were invariably observed to 
be stable to a greater degree than could be ac- 
counted for alone by the use of a locally gen- 
erated carrier, and it was found possible to 
attain entirely satisfactory voice communica- 
tion in areas where the high reverberation level 
prohibited code transmission at ranges of even 


1,000 or 1,500 yards. It is believed that these 
observations can be explained only in terms 
of the averaging effect attending the use of a 
signal having a large number of individual 
components. At any instant one or more of 
these components may be completely obliterated 
due to interference, but it appears that there 
always remain a sufficient number at normal 
level so that on the average the audio signal 
level shows little variation. 

Later tests, utilizing the low-level portions 
of the transmitting circuits working into the 
QB driver indicated results which were in all 
respects equal to those obtained with the spe- 
cially designed power amplifier originally used. 
These tests demonstrated that acceptable per- 
formance can be obtained by using the standard 
echo-ranging driver amplifier with no more 
modification than a careful alignment to insure 
that reasonable operating limits are not ex- 
ceeded. 

Investigation of AM 8-kc Carrier Trans- 
mission 

In order to investigate the potential advan- 
tages of lower-frequency operation, a single 
side-band suppressed-carrier system was con- 
structed to operate at a carrier frequency of 
8 kc."^ For transmitting, this system makes use 
of a 3^2 -inch diameter tubular magnetostric- 
tion transducer 30 inches long and a power 
amplifier capable of delivering 400 watts. The 
amplifier into which the microphone works con- 
tains a limiting circuit to provide for clipping 
the energy of vowel sounds with respect to that 
of the consonants thus permitting a greater 
effective power output.® 

Standard JT sonar equipment with super- 
sonic converter, as discussed in Chapter 10, 
serves as receiver. The transmitting trans- 
ducer, suspended in the water with its axis 
vertical, is nondirectional in the horizontal 
plane, but the 5-foot long JT receiving hydro- 
phone is mounted horizontally and thus is 
highly directive in the horizontal plane. 

Brief tests of this equipment demonstrated 
ranges of about 13,000 yards, approximately 
equal to those obtained with the 26-kc system. 
It is believed that longer ranges were not ob- 
tained because the advantages gained in re- 




PROTOTYPE EQUIPMENT 


191 


duced transmission loss and speech clipping 
were offset by the nondirectional distribution 
of the available power. 

The comparative merits of operation at 8 kc 
and at 26 kc can be completely assessed only 
when the conditions of use are fully specified. 
Greater ranges are possible at 8 kc provided 
the transducers are equally efficient and direc- 
tive, but any transducer having directivity at 
8 kc equal to that of the QB projector at 26 
kc would be of greatly increased, if not prohibi- 
tive, size. The nondirective characteristic is of 
advantage in establishing initial contact with 
other vessels. Consequently, this type of trans- 
mission pattern is well suited for one-way com- 
munication from a controlling vessel to several 
others disposed over a wide arc. 

DIRECTIONALITY OF 
PROJECTORS 

In both the frequency-modulated and the am- 
plitude-modulated systems using components of 
the WCA-2 echo-ranging equipment, the trans- 
mitting and receiving transducers are highly 
directional at the 26-kc carrier frequency used. 
Such directional beams have decided signal- 
strength advantages. During transmission, the 
intensity on the axis exceeds the intensity on 
any bearing from a nondirective transducer of 
equal acoustic output by more than 20 db, and 
in reception the signal-to-noise ratio is simi- 
larly improved by about 20 db. A disadvantage 
of using directional beams lies in the possible 
difficulty of establishing contact between ves- 
sels which do not accurately know their posi- 
tions with respect to each other. During the 
tests the relative position of the two vessels was 
generally known with considerable accuracy, 
consequently no difficulty was experienced from 
this cause. 

15.11 PROTOTYPE EQUIPMENT 

In the construction of prototype equipment 
to be installed on submarines for field opera- 
tion, the design of the 26-kc system first tested 
extensively on surface ships was closely fol- 
lowed. A schematic diagram of the transmit- 
ting circuits used is shown in Figure 5. The 
circuit, including V-101, constitutes a fixed- 


frequency oscillator for generating the carrier 
wave*^ which is impressed through a buffer 
stage on the modulator. In the balanced-ring 
modulator circuit utilizing varistor CR-101, 
potentiometer R-148 is employed to make pos- 
sible the necessary balance upon which ade- 
quate suppression of the carrier and the audio 
frequencies depend. 

Speech signal from the microphone, after 
amplification by a two-stage, resistance- 
coupled, audio amplifier, is impressed upon the 
varistor modulator through transformer T-107. 
The signal from the modulator is matched in 
impedance to the input of a band-pass filter 
Z-101, whose circuit is shown in detail in Fig- 
ure 6. This filter passes the upper side-band 
frequencies, but strongly attenuates the car- 
rier, the lower side band, and all other fre- 
quencies. The remainder of the circuit provides 
amplification of the frequency band passed by 
the filter. Tube V-103 acts as an inverter for 
driving the final stage which consists of beam 
power tubes in parallel push-pull. The amplifier 
is capable of delivering approximately 100 
watts of undistorted power to the QB trans- 
ducer. Figure 3 is a photograph of the proto- 
type model of the transmitting equipment. 

The audio signal used to modulate the car- 
rier covers the 300- to 3,000-c band for the 
upper side band selected for transmission. 
Therefore, it occupies the region between 26.3 
and 29 kc. At the receiving end this band is 
delivered to the input of the WCA-2 echo-rang- 
ing receiver^ where, after passing through the 
r-f tuned circuits and r-f amplification stage, it 
is combined in the first modulator with a 
heterodyne carrier of 87.6 kc. The difference 
frequencies thus produced lie between 58.6 and 


“ Because of the likelihood of encountering doppler 
shifts, the method of generating the transmitter carrier 
employed by the WCA-2 equipment cannot be used for 
voice-modulated underwater telephony. In WCA-2 the 
carrier is obtained by intermodulation between the out- 
put of a fixed-frequency oscillator and a tone from the 
first heterodyne oscillator of the echo-ranging receiver, 
thus permitting the operating frequency of the system 
to be changed over a considerable range by adjustment 
of the receiver oscillator. If this means of generating 
the carrier were used for underwater telephony be- 
tween moving vessels, it is evident that any adjustment 
giving satisfactory transmission in one direction would 
be incorrect for transmission in the other. It is neces- 
sary, therefore, to generate a transmitting carrier 
which is independent of the receiver oscillator. 




192 


SUBMARINE COMMUNICATION SYSTEMS 


OSC TEST 



Figure 5. Schematic of transmitting" circuits used in prototype equipment for underwater telephony. 


61.3 kc, a band within the limits of the pass 
band of the i-f stages of the amplifier. After 
passing through the i-f amplifier, the 58.6- to 
61.3-kc band is impressed upon the second 


modulator whose oscillator frequency is ad- 
justed to 61.6 kc. The difference frequencies 
produced in this case cover the range 300 to 
3,000 c and the original audio signal is thus 



CONCLUSIONS 


193 


recovered without the use in the receiver of a 
carrier identical to that at the transmitter. 
Effectively, however, this 26-kc carrier fre- 
quency appears as the difference between the 
87.6- and 61.6-kc frequencies of the oscillators 
in the receiving amplifier modulators. 

In operation, if the second receiving ampli- 
fier oscillator is set accurately at 61.6 kc, the 
proper relation between the frequency of this 
carrier and the pass band of the i-f stages is 


1. Any equipment designed for the purpose 
of providing underwater communication by 
voice modulation of a supersonic wave should 
employ the single side-band suppressed-carrier 
method of transmission. 

2. Because attenuation of sound in the water 
is unlikely to average less than about 5 db per 
thousand yards at stahdard echo-ranging fre- 
quencies of 26 to 30 kc, increasing the power to 
the projector can in itself effect only slight ex- 



insured. Thus, the frequency of the first oscil- 
lator may be adjusted to compensate for any 
effective doppler shift, giving distortionless 
recovery of the audio signal. With the first 
oscillator set to give a difference of 26 kc be- 
tween its frequency and that of the second 
oscillator, it is found that incoming signals are 
always intelligible. The quality of the recovered 
speech signal then indicates the necessity for 
any final adjustment of the first oscillator. 

Sea trials of two units of this equipment, 
installed on submarines at Pearl Harbor,^ re- 
sulted in consistent ranges up to 14,000 yards, 
with performance generally superior to that 
obtained in earlier tests in Long Island Sound. 
On one occasion, with exceptional thermal 
gradient conditions, satisfactory communica- 
tion was possible at a range of 17,000 yards. 

CONCLUSIONS 

As a result of experience gained with both 
the frequency-modulated and amplitude-modu- 
lated systems, a number of conclusions may be 
drawn with reasonable assurance. 


tensions of the 10,000- to 15,000-yard ranges 
obtainable with the single side-band sup- 
pressed-carrier equipment tested. 

3. If ranges greater than those obtainable 
with the present gear are to be sought, the most 
promising approach appears to be through a 
reduction in carrier frequency. 

4. In regions where underwater reverbera- 
tion prevents intelligible c-w contact with 
standard echo-ranging equipment, supersonic 
carrier telephony may provide a satisfactory 
means of communication. 

5. The maximum range of code signaling 
under good conditions is generally no greater 
than that at which satisfactory underwater car- 
rier telephony can be maintained. 

6. In order to provide suitable underwater 
telephony equipment for use by submarines 
equipped with standard WCA echo-ranging 
gear, only the equipment necessary for deliver- 
ing low-level signals to the input of the driver 
need be provided. 







GLOSSARY 


AFC. Automatic frequency control. 

ARSB. Anchored radio sono buoy. 

ASW. Antisubmarine warfare. 

ATF. Automatic target follower. 

Baffle. A shield used to modify an acoustic path. 
BDI. Bearing deviation indicator. 

Binaural Listening. A method of finding relative 
bearing between own ship and target by comparing, 
for different orientation of the hydrophone axis, dif- 
ferences in phase and arrival time of energy re- 
ceived at two separate channels from a single source. 
Cavitation. The formation of vapor or gas cavities 
in water caused by sharp reductions in local pressure. 
Crystal Tranducer. A transducer which utilizes 
piezoelectric crystals, usually Rochelle salt, ADP, 
quartz, or tourmaline. 

CUDWR. Columbia University, Division of War Re- 
search. 

CW. Continuous w^ave. 

DDI. In this volume, direct deviation indicator. 

Dome. A transducer enclosure, usually streamlined, 
used with echo-ranging or listening devices to mini- 
mize turbulence and cavitation noises arising from 
the transducer’s passage through the water. 

DRSB. Directional radio sono buoy. 

ERSB. Expendable radio sono buoy. 

HUSL. Harvard Underwater Sound Laboratory. 
Hydrophone. An underwater microphone. 

JK. Navy designation for a listening system using a 
large crystal hydrophone. 

Letter Articulation. A measure of the efficacy of 
a communications system, expressed as the percentage 
of correctly-heard sounds. 

Listening Differential. Ratio of.the just-perceptible 
change in level to the initial signal level. 
Magnetostriction Effect. Phenomenon exhibited by 
certain metals, particularly nickel and its alloys, 
which change in length when magnetized, or (Villari 
effect) when magnetized and then mechanically dis- 
torted, undergo a corresponding change in magnetiza- 
tion. 

MG. Motor-generator. 

MIT-USL. Massachusetts Institute of Technology, Un- 
derwater Sound Laboratory. 

MVP. Merchant vessel protection. 

NLL. New London Laboratory. 

NLM. Noise level monitor. 

Projector. An underwater acoustic transmitter. 

QB. Standard Navy searchlight-type echo-ranging 
equipment using Rochelle salt transducers. 

QC. Navy designation for standard echo-ranging gear 
using a magnetostriction projector. 

Radar. Generic term applied to methods and apparatus 
that use RAdio for Detection And Ranging. 


Range Rate. Rate of change of range between own 
ship and target. 

Recognition Differential. The number of db by 
which a signal must exceed the background in order 
to be recognized 50 per cent of the time. 
Reverberation. Sound scattered diffusely back to- 
wards the source, principally from the surface or 
bottom and from small scattering sources in the 
medium such as bubbles of air and suspended solid 
matter. 

Reynolds Number. A nondimensional ratio used for 
comparing the conditions for similar motions in 
fluids. The ratio of any typical length of a body times 
its velocity, to the kinematic coefficient of viscosity of 
the fluid. 

/)C-Rubber. a rubber compound with the same pc (den- 
sity X velocity of sound) product as water. 

RLI. Right-left indicator. 

Rochelle Salt (KNaC 4 H 40 (r 4 H 20 ). Potassium so- 
dium tartrate, a piezoelectric crystal used in sonar 
transducers. 

Shadow Zone. Region in which refraction effects cause 
exclusion of echo-ranging signals. 

Slicks. Oils, dyes, metal powders, etc., used to mark 
an area on the water surface. 

Sonar. Generic term applied to methods or apparatus 
that use SOund for NAvigation and Ranging. 

Sonic Frequencies. Range of audible frequencies, 
sometimes taken as from 0.02 to 15 kc. 

Supersonic Frequencies. Range of frequencies higher 
than sonic. Sometimes referred to as ultrasonic to 
avoid confusion with growing use of the term super- 
sonic to denote higher-than-sound velocities. 

Target Aspect. Orientation of the target as seen from 
own ship. 

TDC. Torpedo data computer. 

TDM. Torpedo detection modification. 

TDS. Target designation system. 

Threshold of Feeling. Level at which increasing 
sound intensity becomes painful to the listener. 

TLR. Triangulation listening ranging. 

Towing Angle. Angle between the towing cable and 
the normal to the water’s surface. 

Transducer. Any device for converting energy from 
one form to another (electrical, mechanical, or acous- 
tic). In sonar, usually combines the functions of a 
hydrophone and a projector. 

UCDWR. University of California, Division of War 
Research. 

X-CUT. A cut in which the electrode faces of a piezo- 
electric crystal are perpendicular to an X-, or elec- 
trical, axis. 

Yaw. Angular deviation from line of course taken 
in a horizontal plane about the vertical axis of the 
hydrophone housing. 




i9r> 


<4 • 



BIBLIOGRAPHY 


Numbers such as Div. 6-G23-M6 indicate that the document listed has been microfilmed and that its title 

mnrconsuh th T"' ’ ‘w “> ‘"dex volume and to the micro- 

him, consult the Army or Navy agrency listed on the reverse of the half-title page. 


^ (Chapter 1 

1. Detection of Aircraft by Listening from Submarines, 
Ralph C. Maninger and Edward Gerjuoy, NDRC 

6.1- srll28-2221, Report P33/R1437, NLL, May 31, 

Div. 6-623-M6 

2. Discussion at a Conference on Listening Techniques 
Held at Neiv London on March 10, 19 AS, William B. 
Snow, Report G1 / R242, NLL, Mar. 29, 1943. 

Div. r)-621-M2 

Chapter 2 

1. Binaural Listening System, Donald P. Loye, NDRC 

6.1- sr20-565, Report P12/R145, NLL, Jan. 12, 1943. 

Div. 6-621-Ml 

("Iiapler 3 

1. Sonic Listening Aboard Submarines, OSRD 5311, 
NDRC 6.1-srll31-1885, Service Project NS-140, 
CUDWR-SSG, February 1945. Div. 6-623. 1-M8 

2. Basic Factors Affecting the Perforynance of Sonic 
Listening Gear on Submarines, OSRD 5031, NDRC 

6.1- srll31-1888, Service Project NS-140, CUDWR- 

SSG, February 1945. Div. 6-623.1-M9 

3. Submarine Listening Systems, Report of Confer- 
ence, September 29, 19^3, Carlton R. Sawyer, Re- 
port D24/P30/560, NLL, Oct. 26, 1943. 

Div. 6-623.1-M3 

4. Conference on Submarine Sound Equipment, Wil- 
liam B. Snow, Report P32/R608, NLL, Nov. 8, 1943. 

Div. 6-623-M5 

(3iapter 4 

1. Measurements of Projector and Hydrophone Per- 

formance, Definitions and Terms, Eginhard Dietze 
and Joseph B. Keller, NDRC 6.1-srll30-1833, Sept. 
19, 1944. Div. 6-551-M12 

2. Survey of Underwater Sound. Sounds from Subma- 
Hnes, Vern O. Knudsen, R. S. Alford, and J. W. Em- 
ling, 6.1-NDRC-1306, Report 2, Dec. 31, 1943. 

Div. 6-580. 1-M2 

3. Listening Systems for Patrol Craft, NDRC 6.1- 
sr692-1698, BTL, Dec. 1, 1944. Div. 6-622.1-M5 

4. Electrical Equipment for Patrol Craft Listening 
Systems, OSRD 4846, NDRC 6.1-sr346-1322, Dec. 

1944. Div. 6-622.1-M4 


5. Comparative Field Tests of Undertvater Listening 
Equipment Installed bn the Elcobel, Walter F. 
Graham and Ralph C. Maninger, NDRC 6.1-srll28- 
1569, Report P33/R862, NLL, Sept. 30, 1944. 

Div. 6-622. 1-M3 

6. Equipment Developed and Used on the Amada for 
Underwater Sound Investigations, Walter F. 
Graham, Report P33/R1379, NLL, Feb. 28, 1945. 

Div. 6-621-M6 

7. Proposed Listening Tests, William B. Snow, Report 

G1/R407, NLL, May 22, 1943. Div. 6-621-M3 

8. Experimental Investigation of Factors Involved in 
Sonic Listening, Ralph C. Maninger, NDRC 6.1- 
srll28-1932. Report P33/R1319, NLL, Feb. 28, 1945. 

Div. 6-621-M7 

Chapter 5 

1. Submarine and Surface Craft Listening Equipment, 
Donald P. Loye and Ralph C. Maninger, NDRC 

6.1-sr20-1020, Report D24/D38/R391, Service Proj- 
ect NS-113, NLL, Sept. 10, 1943. Div. 6-623-M4 

2. Fundamental Listening Studies at the New London 
Laboratory, Ralph C. Maninger, NDRC 6.1-srll28- 
2210, Report P33/R1409, NLL, May 30, 1945. 

Div. 6-621-M8 

3. The Use of the CK Tube as an Antisubmarine 

Listening Device, Irving Langmuir and E. F. Hen- 
nelly, GE, Feb. 15, 1943. Div. 6-622.1-Ml 

Chapter 6 

1. Textbook of Sound, Second Edition, A. B. Wood, 
p. 234. 

2. Phase-Actuated Locator, OSRD 1897, NDRC 6.1- 
sr695-997, BTL, Aug. 30, 1943. Div. 6-622.1-M2 


Chapter 7 

1. The JP Overside Directive Sonic Listening Equip- 
ment for Small Patrol Craft, Russell O. Hanson, 
NDRC C4-sr20-541, Report D22.2/3975, Service 
Project NS-113, NLL, Nov. 6, 1942. 

Div. 6-622.2-M4 

2. The JP Overside and Through-the-Hull Directive 

Sonic Listening Equipment for Small Patrol Craft, 
Russell O. Hanson and Edwin E. Teal, OSRD 4744, 
NDRC 6.1-srll28-1928, Report D22/D38/R1310, 
NLL, Feb. 7, 1945. Div. 6-622.2-M5 




197 


198 


BIBLIOGRAPHY 


3. The Straight Toroidally Wound Plastic-Covered 

• Magnetostrictioii Hydrophone, Wilbur T. Harris, 

NDRC 6.1-srll28-1573, Report G12/R804, NLL, 
June 15, 1944. Div. 6-612.62-M28 

4. Directive Overside Listening Gear for Small Patrol 
Craft, J. Warren Horton, Report D22/3268, Service 
Project NS-113, NLL, June 16, 1942. 

Div. 6-622.2-Ml 

5. Maintenance Manual for JP Sonic Listening Equip- 
ment, May'k II, Report D22/3783, NLL, Sept. 1, 1942. 

Div. 6-622.2-M2 

6. Deep and Shallow Water Tests on D22-JP Listening 

Equipment, NDRC C4-sr623, NLL and UCDWR, 
Nov. 3, 1942. Div. 6-622.2-M3 

7. Sonic Listening and Recordings of Sounds from the 

USS Balao during Deep Submergence Tests, April 
27 19US, Edwin E. Teal, Report G1/R333, NLL, 
May 12, 1943. Div. 6-623-M3 

8. Through-the-Hull Sonic Listening Equipment, Ed- 
win E. Teal, Report D38/R155, NLL, Apr. 22, 1943. 

Div. 6-622.3-Ml 

9. Comparative Listening Tests of Through-the-Hull 
Sonic and Syipersonic and QBG Sonic and Super- 
sonic Listening E quipment, Ralph C. Maninger and 
A. Kenneth Tatum, OSRD 1593, NDRC 6.1-sr20-790, 
Report D38/R374, NLL, NS-113, June 23, 1943. 

Div. 6-622.3-M3 

10. Deep Water Tests of the Through-the-Hull Sonic 

Listening Eqyiipment, T. F. Johnston, Report U-73, 
NLL, June 18, 1943. Div. 6-622.3-M2 

11. Comparative Field Tests of Underwater Listening 
Equipment Installed on the Elcobel, Walter F. 
Graham and Ralph C. Maninger, NDRC 6.1-srll28- 
1569, Report P33/R862, NLL, Sept. 30, 1944. 

Div. 6-622.1-M3 

12. A Permanent Magnet Magnetostriction Hydrophone 

Constructioji, Wilbur T. Harris, NDRC 6.1-srll28- 
1921, Report G12/R1248, Service Project NS-102, 
NLL, Dec. 20, 1944. Div. 6-612.62-M41 

13. Installation, Operation, Maintenance of JP-1 Sound 
Receiving Equipment. Topside Sonic Listening 
Equipment, Report D24/R417, NLL, Sept. 1, 1943. 

Div. 6-623.1-Ml 

14. Experimental Investigation of Factors Involved in 
Sonic Listening, Ralph C. Maninger, NDRC 6.1- 

• srll28-1932. Report P33/R1319, NLL, Feb. 28, 1945. 

Div. 6-621-M7 

Chapter 8 

1. Depth of Towed Fish and the General Curve of a 
Towing Cable in Water or Air, Calvin R. Gongwer, 
NDRC 6.1-sr20-659, Report G2/R238, Service Proj- 
ect NA-107, NLL, June 25, 1943. Div. 6-624.3-M7 


2. Measurements on the HW-2 Hydrophone, Sweet- 
water Calibration Station, UCDWR, Jan. 28, 1944. 

3. Sonic Detection of an Airplane from a Submarine, 
L. J. Sivian, UCDWR, Dec. 26, 1941. Div. 6-623-Ml 

4. Towed Hydrophones for Blimp Use, Russell I. 
Mason, Report P2/2767, NLL, May 9, 1942. 

Div. 6-624.3-Ml 

5. Towed Hydrophones for High Speeds as Used at 
New London Laboratory, Dick P. Fullerton, Jr., 
Report D25/3864, NLL, Aug 28, 1942. 

Div. 6-624.3-M8 

6. The Use of Hydrojjhones Towed from Dirigibles, 

J. Warren Horton, Report D25.2/4178, NLL, Oct. 7, 
1942. Div. 6-624.3-M2 

7. Blimp-Towed Hydrophones, William H. Fritz, Re- 
port D25/4423, Service Project NA-107, NLL, May 

7, 1943. Div. 6-624.3-M3 

8. Transmission of Sound Along Wakes, NDRC 6.1- 
srl046-1054. Service Project NS-141 and MIT Re- 
search Project DIC-6187, MIT, July 26, 1944. 

Div. 6-540.32-M4 

9. HW Towed Hydrophone, NDRC 6.1-srl046-1679, 
Service Project NA-107 and MIT Research Project 
DIC-6187, MIT-USL, Dec. 1, 1944. 

Div. 6-624.3-M4 

10. Blimp-Towed Hydrophones, William H. Fritz, 

OSRD 4450, NDRC 6.1-sr20-1920, Report 
D25/R1240, Service Project NA-107, NLL, Dec. 
1, 1944. Div. 6-624.3-M5 

11. Towed Microp>hones for Blimps, William H. Fritz, 

Report D25/3395, NLL. Div. 6-624.3-M6 


Chapter 9 

1. The Directional Radio Sono Buoy, OSRD 5279, 
NDRC 6.1-srll28-2224, Report D34/R1200, Service 
Projects NS-106 and NS-198, NLL, May 20, 1945. 

Div. 6-624.2-M7 

2. Installation and Maintenance Instructions for 

Radio Receiving Equipment AN/ARR-16, Report 
D34/R1167, Service Project NS-106, NLL, Nov. 1, 
1944. Div. 6-624.2-M4 

3. Operator’ s Manual for the Directional Radio Sono 

Buoy, Report D34/R1260, Service Project NS-330, 
NLL, Dec. 15, 1944. Div. 6-624.2-M6 

4. Tests of Subaqueous Microphones for Sono Radio 

Buoys, J. Warren Horton and William B. Snow, 
NDRC C4-sr20-055, Report D3/1908, NLL, Jan. 27, 
1942. Div. 6.1-625.2-Ml 

5. Desirable Characteristics for Expendable Radio 
Sonic Buoy System, William D. Neff, Report 
D16/R156, NLL, Jan. 28, 1943. Div. 6.1-624.11-M2 


• 


BIBLIOGRAPHY 


6. Operation and Use of the Expendable Radio So 7 iic 
Buoy Equipment, NDRC 6.1-sr20-657, Report 
D16/R188, NLL, Apr. 13, 1943. Div. 6-624.11-M3 

7. Expendable Radio Sonic Buoy ayid Magnetic Air- 
borne Detection Submarine Search Tests, April 23, 
19 is, Lakehurst, New Jersey, Russell I. Mason, Re- 
port D16/R320, NLL, May 5, 1943. Div. 6-624.12-Ml 

8. Comparative Listeyiing Tests, William D. Neff, Re- 
port D16, R362, NLL, May 28, 1943. 

Div. r)-624.12-M2 

9. Expendable Radio Sonic Buoy Pattern Operation 
with Single Frequency and Multiple Frequency 
Methods, AN /CRT-1 and AN / ARR-3 Equipments, 
Joseph A. Barkson, Report D34/R378, Service Proj- 
ect NS-106, NLL, June 4, 1943. Div. 6-624.12-M3 

10. Necessay'y AN/CRT-1 Electrical hnprovements. Ex- 

pendable Radio Sonic Buoy, Henry N. Jasper, Jr. 
and Walter L. Clearwaters, Report D16/R381, NLL, 
June 12, 1943. Div. 6-624.11-M4 

11. Expendable Radio Sono Buoy and Magnetic Air- 
bomie Detection Submarine Search Tests, Russell V. 
Lewis, Report D16/R438, NLL, July 23, 1943. 

Div. 6-624.12-M4 

12. Proposed Changes in DIG Mark IV -E Hydrophone, 

Robert R. MacLauphlin, Report D16/R467, NLL, 
Aug. 3, 1943. Div. 6-612.62-M13 

13. Maintenance Instructions for Radio Trayismitting 
Equipment AN /CRT /I A, Report D16 R912, Service 
Project NS-106, NLL, May 12, 1944. 

Div. 6-624.11-M5 

14. The Expendable Radio Sono Buoy, OSRD 4115, 
NDRC 6.1-srll28-1581, Report D16/R1035, Service 
Projects NS-106 and NS-198, NLL, July 27, 1944. 

Div. 6-624.1-Ml 

15. Representative Performance Characteristics Mark 
IV -E ERSB Hydrophoyies, Joseph A. Barkson and 
Robert R. MacLaughlin, Report D16/R685, Service 
Project NS-106, NLL, Aug. 9, 1944. 

Div. 6-624.12-M6 

16. Expendable Radio Sono-Buoy, Use with Echo-Rang- 

ing Equipment, Price E. Fish, Report D16/R1368, 
NLL, Feb. 20, 1945. Div. 6-624.12-M7 

17. Operation and Use of the Expeyidable Radio-Sonic 
Buoy, Model D-16 Mark IV D, NDRC C4-sr20-359, 
Report D16.9/4404, NLL. 

18. Research and Engineering Department Report on 
S. B. Tests at New London, May 12 to May 14, 
W. J. Brown, Report D3-2897, Brush Development 
Co., NLL, May 19, 1942. 

19. Method of Adding Directional Charactei'istics to the 
Radio Sonic Buoy, Russell 1. Mason, Report 
D34/R281, NLL, May 3, 1943. Div. 6-624.2-Ml 


199 


20. Conference on D-34 Listening Tests, Ralph C. 
Maninger, Report D34/R372, NLL, June 2, 1943. 

Div. 6-624.21-M2 

21. Binaural No7i-Rotating Directional Radio Sonic 
Buoy System, Elliott J. Lawton, OSRD 1878, NDRC 
6.1-sr323-1108, GE, Aug. 16, 1943. Div. 6-624.2-M2 

22. Mahitenayice histmictions for Radio Transmittmg 

Equipynent AN /CRT-4 (XN-1), prepared by Co- 
lumbia University for US Navy Department 
BuShips, Report D34/R1169, Service Project NS- 
106, NLL, Nov. 15, 1944. Div. 6-624.2-M5 

23. Effects of Airplane Noise on Listenmg with Head- 
phones, Experiments Conducted at the Harvard 
Psycho- Acoustic Laboratory, September 27-29, 1942 
and November G-8, 1942, William B. Snow and 
William D. Neff, NDRC 6.1-sr20-550, Report 
D16/D34/R107, CUDWR-NLL, Jan. 25, 1943. 

Div. 6-580.3-Ml 

24. Db'ectional Radio Sono Buoy CoJiference and Dem- 
onstration, ASDevLant, Quonset, Naval Air Station, 
March 22, 1944, Joseph A. Barkson, Report 
D34/R747, NLL, Mar. 29, 1944. Div. 6-624.2-M3 

25. Headphoyie Comparison Tests, Merritt B. Jones and 
William B. Snow, Report G27/R952, NLL, June 1, 

1944. Div. 6-624.12-M5 

26. Listening Tests on May 14, 1943 of Hydrophoyie for 

Directioyial Sonic Buoy, Ralph C. Maninger, Report 
D34/R349, Service Project AG-55, NLL, May 19, 
1943. Div. 6-624.21-Ml 

27. Headphones for Expendable Radio Sonic Buoy 
Operators, William B. Snow, NDRC 6.1-sr20-633, 
Report D6/R152, NLL, Jan. 21, 1943. 

Div. 6-624.11-Ml 

Chapter 10 

1. Developynent of the Directional Voice Frequency 

Toroidal MagnetosU'iction Hydrophone, Arthur L. 
Thuras, OSRD 775, NDRC C4-sr20-214, Report 
G5S/3413, NLL, July 1, 1942. Div. 6-554.2-M3 

2. Development of Magnetosty'iction Hydrophones, 

July 1, 1942 to April 1, 1943, Arthur L. Thuras, 
NDRC 6.1-sr20-639, Report G12/R158, NLL, Apr. 1, 
1943. Div. 6-612.62-M6 

3. Model Tests of JP-1, 5-Foot Hydrophone Baffle, 
Leslie J. Hooper, Report G2/6116, NLL, January 

1945. Div. 6-623.1-M7 

4. The Model JT Sonar Equipment, Carlton R. Sawyer, 

OSRD 5275, NDRC 6.1-srll28-2215, NLL, May 25, 
1945. Div. 6-623.2-M7 

5. Observations and Operations of Ship's Noise Aboard 
the USS Balao, April 27, 1943, R. York Chapman, 
Report D24/R314, NLL, May 5, 1943. Div. 6-623-M2 




200 


BIBLIOGRAPHY 


6. Transmission of Noise Through J P-1 Training Gear, 
Hollie C. Williams, Report D24/R550, NLL, Oct. 14, 

1943. Div. 6-623.1-M2 

7. Discussion of Balance Requirements of Hydrophones 

Used with an RLI-Type of Bearing Deviation Indi- 
cator, William F. Arndt, Report D51/D55/R845, 
NLL, Apr. 10, 1944. Div. 6-623.3-M3 

8. A Permanent Magnet Magnetostrictions Hydro- 
phone Construction, Wilbur T. Harris, NDRC 6.1- 
srll28-1921. Report G12/R1248, NLL, Dec. 20, 1944. 

Div. 6-612.62-M41 

9. Hydrophone Tests Adopted at the New London 
Laboratory of Columbia University , Division of War 
Research, William B. Snow, NDRC 6.1-srll21-1849, 
Report G12/R1092, NLL, Sept. 1, 1944. 

Div. 6-612.51-Mll 

10. The Modified Baffle for Topside Straight Hydro- 

phones, James W. Follin, Jr., Report G12/R1010, 
NLL, Aug. 12, 1944. Div. 6-612.62-M32 

11. The Model JP-1 Sound Receiving E q 2 iipment, Hollie 

C. Williams, NDRC 6.1-srll28-1033, Report 
D24/540, Service Project NS-113, NLL, Mar. 16, 

1944. Div. 6-623.1-M4 

12. Some Investigations of Isolation Mounts for JP-1 

Sotmd Receiving E quipment, Mark Harrison, Report 
D24/R750, Service Project NS-113, NLL, Mar. 10, 
1944. Div. 6-623.1-Mll 

13. Analysis of Bearing Deviation Indicator (BDI) 

Systems, William F. Arndt, Report D51/R823, NLL, 
Mar. 22, 1914. Div. 6-623.3-M2 

14. Interference and Its Effects on the RLI Indication, 

Frederick C. Reed, Jr., Report D51/D55/R1405, 
NLL, Feb. 28, 1945. Div. 6-623.3-M4 

15. Preliminary Ojjerating histructions for the JT 
Sonar Eq^iipmeiit, Report D55/R1229-A, Service 
Projects NS-113 and NS-337, NLL, December 1943. 

Div. 6-623.2-Ml 

16. Op>eratoPs Manual for the Model JT Sonar Equip- 
ment, Report D55 R1401, NLL, Feb. 28, 1945. 

Div. 6-623.2-M6 

17. Modification of Submarine Sonar Equipment, Playi 

III, RLI Applied to QB Projector with JP-1 and 
QC-JK Oj)erator m Forward Torpedo Room, Wil- 
liam F. Arndt, Ogden E. Sawyer, Report D55/R824, 
Service Projects NS-113 and NS-142, NLL, Mar. 24, 
1944. Div. 6-623.2-M4 

18. Modification of Submarme Sonar E quij^ment. Plan 
II, RLI AjjjAied to QB Projector with JP-1 Listen- 
ing Gear Installed in Conning Tower, William F. 
Arndt, Ogden E. Sawyer, Report D55/R825, Service 
Projects NS-113 and NS-142, NLL, Mar. 23, 1944. 

Div. 6-623.2-M2 


19. Modification of Submarine Sonar Equip 7 nent, Plan I, 
RLI and Power Training for JP-1 Gear, William F. 
Arndt, Ogden E. Sawyer, Report D55/R826, Service 
Projects NS-113 and NS-142, NLL, Mar. 24, 1944. 

Div. 6-623.2-M3 

20. Grajyhical Evaluation of the Effect on RLI Accuracy 
of an Interfering Sig^ial and a Study of the Relative 
Merits of the Two-Section, 5-Foot Hydrophone vs 
the Ten-Section, P.M. 5-Foot Lobe Reduction Hydro- 
pho7ie from an Interference Viewpoint Frontal Lobe 
Section Only, Frederick C. Reed, J. B. Haymes, and 
L. Marts, Report D55/R1144, Sept. 22, 1944. 

Div. 6-612.5-Ml 

21. Difference Listening as an Aid to the Right-Left 

hidicator in the D-55 System, Robert J. Callen, Re- 
port D55/R1187, NS-113 and NS-330, NLL, Oct. 17, 
1944. Div. 6-623.2-M5 

22. Installatioyi, Opejmtion, and Mamtenance of JP-1 

Sound Receiving Equipynent. Topside Sonic Listen- 
ing Equipment, Report D24/R417, Service Project 
NS-113, NLL, Sept. 1, 1943. Div. 6-623.1-Ml 

23. Prelimmai'y Opey'ating InsUuctioyis for the JT 
Sonar Equipment, Report D55/R1229-A, Service 
Projects NS-113 and NS-337, NLL, December 1943. 

Div. 6-623.2-Ml 

24. Mainteyiance and Trouble-Shooting Instructions for 

Models JP-1, JP-2 and JP-3 Sound Receiving Equip- 
ment, NDRC 6.1-srll28, Report D24/R837, Service 
Project NS-113, NLL. Div. 6-623.1-MlO 

25. Pey'moflux Headset for JP Series Equipmeyit, Wil- 

liam B. Snow, Report D24/R1128, Service Project 
NS-113, NLL, Sept. 14, 1944. Div. 6-623.1-M6 

26. Tests of JP-1 Equipment with 3 -Ft, Jf-Ft, and 5-Ft 
Hydrophones and QB Sound Gear, Walter F. 
Graham, Report P33/R1031, NLL, July 13, 1944. 

Div. 6-623.1-M5 

Chapter 1 1 

1. 692 Submarme Soyiar, OSRD 6633, NDRC 6.1-sr692- 

2396, BTL, Feb. 28, 1946. Div. 6-633.3 Ml 

2. Underwater Sound Measiming Station at Birchwood 
Lake, NDRC 6.1-sr346-1332, BTL, July 27, 1945. 

Div. 6-621-M9 

3. Status Report Under Task 2 on Blast Tests of Toj)- 
side Projectors, US Navy, Bureau of Ships Sonar 
Development. 

Chapter 12 

1. PrelUninary Investigation for a Proposed Dual 
Listening System, Charles J. Loda and J. Kneeland 
Nunan, Report P30/R519 [for the] period from 
August 9 to September 9, 1943, NLL, Sept. 13, 1943. 

Div. 6-623.3-Ml 


BIBLIOGRAPHY 


201 


2. Analyses of Bearing Deviation Indicator (BDI) 

Systems, William F. Arndt, Report D51/R823, NLL, 
Mar. 22, 1944. Div. 6-623.3-M2 

3. Instruction Guide for Sperry Equiptnent Used in 

the Triangulation-Listening -Rang mg Systeyn, Re- 
port D51 5846, NLL. Div. 6-623.3-M8 

4. Mechanical Features of JAA Trainmg System, Cal- 

vin A. Gongwer, Report D51/R1407, Service Project 
NS-247, NLL, Feb. 28, 1945. Div. 6-623.3-M5 

5. Submarine Triangulation-Listening-Ranging Sys- 

tem, Carlton R. Sawyer, OSRD 5291, NDRC 6.1- 
srll28-2218. Report D51 R1429, Service Project 
NS-247, NLL, May 28, 1945. Div. 6-623.3-M7 

6 . Computations on Bearing Deviation Indicator Re- 
sponse, Edward Gerjuoy and Edward S. Seeley, Re- 
port D51/R765, NLL, Feb. 21, 1944. 

Div. 6-631.41-M5 

7. Discussion of Balance Requirements of Hydrophoyies 

Used with an RLI-Type of Bearing Deviation Indi- 
cator, William F. Arndt, Report D51/D55/R845, 
NLL, Apr. 10, 1944. Div. 6-623.3-M3 

8 . Interference and Its Effects on the RLI hidication, 

Frederick C. Reed, Jr., Report D51/D55/R1405, 
Service Projects NS-247 and NS-337, NLL, Feb. 28, 
1945. Div. 6-623.3-M4 

9. Discussion of Electronic Equipment for Triangu- 

lation-List enmg-Rangmg System on USS Conger, 
Leonard W. Nosker and Richard G. Stephenson, 
Report D51/R1422, Service Project NS-247, NLL, 
Feb. 28, 1945. Div. 6-623.3-M6 

10 . A Permanent Magnet Magnetostriction Hydrophone 
Construction, Wilbur T. Harris, NDRC 6.1-srll28- 
1921, Report G12/R1248, NLL, Dec. 20, 1944. 

Div. 6-612.62-M41 

Chapter 13 

1 . Tests on the Electro-Protective Corporation Tor- 

pedo Detector, William B. Snow and Edwin E. Teal, 
NDRC 6.1-srH28-1582, Report P20/R1050, NLL, ' 
Nov. 15, 1944. Div. 6-626.1-M5 

2 . Merchant Vessel Protection, Soyiic Detection of Tor- 
pedoes from Merchant Shijjs, William B. Snow and 
Don A. Proudfoot, OSRD 3265, NDRC 6.1-srH28- 
1253, Report P20/R688, NLL, Feb. 3, 1944. 

Div. 6-626.1-M2 

3. Additional Torpedo Noise Measurements, Edwin E. 
Teal, Report P20/R1022, NLL, July 12, 1944. 

Div. 6-626.1-M4 

4. Listening Studies Using QBF and JK Transducers, 

Donald P. Loye and A. Kenneth Tatum, Report 
G13/R599, Service Project NS-113, NLL, Nov. 9, 

1943. Div. 6-621-M4 


5. Noise Measurements in QBF and JK Streamlined 

Domes, A. Kenneth Tatum, Report G13/R613, NLL, 
Nov. 12, 1943. Div. 6-621-M5 

6 . Status of Merchant Vessel Protection Listening 
Equipment on USS YP252, Edwin E. Teal and 
William B. Snow, Report P20/R977, NLL, July 11, 

1944. Div. 6-626.1-M3 

7. Maintenance Manual for WCA-2 So 7 iar Equipment, 
Bureau of Ships, Nayy Department, NavShips 900, 
414. 

8. The WCA-2 Torpedo Detectioyi Modification, J. 

Warren Horton, Victor V. Graf, and F. T. Schell, 
OSRD 5274, NDRC 6.1-srll28-2216, Report P60/ 
R1427, NLL, May 28, 1945. Div. 6-626.2-M4 

9. Installation, Operation and Mamtenance histruc- 

tions for the Torpedo Detection Modification of 
the WCA-2 Sonar Equijjment, Report P60/R1316, 
NLL, Feb. 15, 1945. Div. 6-626.2-Ml 

10 . Self Noise Measurements with QB Transducer 

with and without Streamlined Do 7 ne, USS Thorn- 
hack, Henry B. Hoff and F. T. Schell. Report P60/- 
R1346, NLL, Feb. 28, 1945. Div. 6-626.2-M2 

11. Load Tests on WCA-2 Traming Equiptnent While 
Operated at Tor 2 )edo Detection Scanning Speeds 
ivith and without 5r Budd Dome, USS Thornback, 
F. T. Schell, Report P60/R1431, NLL, Feb. 28, 

1945. Div. 6-626.2-M3 

12. Notes on Atichored Vessel Screening, W. H. Fritz, 
Report D44/R296, Apr. 23, 1943. 

13. Calibration Tests of Electro Protective Corpora- 
tion’s Torpedo Detector Installed on Tanker Mohil- 
gas, Edwin E. Teal, Sylvester J. Haefner, and Ed- 
ward Gerjuoy, Report P20/R517, NLL, Oct. 13, 

1943. Div. 6-626.1-Ml 

14. Merchant Vessel Protection, An Evaluation of 
Means of Providing Protection for Merchant Ves- 
sels against Torpedo Attack, William B. Snow, Don 
A. Proudfoot, and Edwin E. Teal, NDRC 6.1- 
srH28-1597, Report P20 R1216, NLL, Nov. 30, 

1944. Div. 6-626.1-M6 

Chapter 14 

1. hivestigation of Water Noise Conditions m Chesa- 
peake Bay, May 18 to June 3, 19 U2, Donald P. Loye 
and Don A. Proudfoot, NDRC C4-sr20-221, Report 
D12B/3289, NLL, July 15, 1942. Div. 6-625.11-Ml 

2 . Supplementary Investigation of Water Noise in 
Chesapeake Bay, Donald P. Loye, Don A. Proudfoot, 
and Sylvester J. Haefner, OSRD 828, NDRC C4- 
sr20-235, NLL, Aug. 24, 1942. Div. 6-625.11-M2 

3. A^nhient Noise Survey — Miami Area and the East 

Coast of the United States, Henry B. Hoff, Don- 
ald L. Cole, and Robert A. Wagner, NDRC 6.1- 
srll28-1944. Report D46A/R1215, Service Project 
NO-163, NLL, May 5, 1945. Div. 6-580.33-M3 


202 


BIBLIOGRAPHY 


4. Cahle-Co7inected Hych'ophone Systems, William B. 
Snow, Henry B. Hoff, and Alice M. Berry, OSRD 
5243, NDRC 6.1-srll28-1946, Report D12/R1213, 
Service Project NO-163, NLL, May 21, 1945. 

Div. 6-625.1-M9 

5. Cable-Connected Hydrophones, Block Island In- 
stallation, J, Warren Horton and Michael S. Shane, 
Report D12A/3013, NLL, June 2, 1942. 

Div. 6-625.1-M2 

6. Transmission Survey Block Island Sound, William 
B. Snow, Henry B. Hoff, and J. J. Markham, NDRC 
6.1-srll28-1027, NLL, Mar. 16, 1944. 

7. Chesapeake Bay Cable-Connected Hydrophone Sys- 
tem, Robert A. Wagner, OSRD 1181, NDRC 6.1- 
sr20-566. Report D12B/R115, NLL, Jan. 11, 1943. 

Div. 6-625.1-M3 

8. Chesapeake Bay Project, Cable-Coiinected Hydro- 
phones, J. Warren Horton and Michael S. Shane, 
Report D12B/3193, NLL, May 16, 1942. 

Div. 6-625.1-Ml 

9. Co7np>arative Tests on Block Island Amplifier Sys- 

tem vs Amplifier Tentatively Selected for the 
Chesapeake Bay Cable-Connected Hydrophone 
Installation, Donald P. Loye, Henry B. Hoff, and 
Mark Harrison, Report D12AB/3898, NLL, Sept. 
12, 1942. Div. 6-625.1-M5 

10. Recommended Calib7'atio7is of Block Isla7id Cable 
System, Henry B. Hoff, Report D12A/R1118, Serv- 
ice Project NO-163, NLL, Sept. 7, 1944. 

Div. 6-554-M35 

11. Cape Henry Listening Post — Report of Transmis- 
sion Tests on Line Facilities, H. H. Felder, Report 
3430-HHF-EN, BTL, Jan. 12, 1943. Div. 6-625.1-M7 

12. Loss Reduction Obtainable by Loading 107 Type 
Sub7narine Sig7ial Cable, T. Shaw, Report 3430-TS- 
HHF-VB, BTL, Jan. 12, 1943. Div. 6-625.1-M6 

13. Noise 071 Hydrophone Cable at Cape Henry, Va., 

R. S. Tucker, Report 3410-RST-HP, BTL, Feb. 5, 
1943. Div. 6-625.1-M8 

14. Progra7n Repeater for Chesapeake Bay Cable In- 

stallatio7i, Frank P. Herrnfeld, Report D12B/R195, 
NLL, Mar. 19, 1943. Div. 6.625.1-M4 

15. Pre-emphasis in So7io Radio Buoys, William B. 
Snow, Report D3/2601, NLL, Apr. 22, 1942. 

Div. 6-625.2-M2 

16. Hydrophone Liste7ii7ig Tests, Donald P. Loye and 

Russell O. Hanson, NDRC C4-sr20-090, Report 
G12/2623, NLL, Apr. 30, 1942. Div. 6-625.2-M3 

17. Field Test of Netv Type Sono Radio Buoy Tra7is- 

7nitter in Bosto7i Ha7'bor Area, May 20 to 27 Inclu- 
sive, Robert A. Fox, Report D3/2978, NLL, June 1, 
1942. Div. 6-625.2-M4 


18. Water Noise Tests at Block Island on the JM-1 

Radio Sonic Buoy, Robert A. Fox, Report D3. 2/3977, 
NLL, Sept. 15, 1942. Div. 6-625.2-M5 

19. The A7ichored Radio So7io Buoy, William B. Snow, 
Russell 1. Mason, and Walter F. Graham, OSRD 
4745, NDRC 6.1-srll28-1929, Report D3/R1311, 
Service Project NS-102, NLL, Feb. 10, 1945. 

Div. 6-625.2-M6 

20. Underwater Sou7id Su7'vey New York Harbor Ap- 
proaches, Don A. Proudfoot, NDRC 6.1-sr20-794, 
Report D12E/R453, NLL, Aug. 28, 1943. 

Div. 6-625.11-M3 

21. S7tap Diaphrag7n — Appare7it Mode of Operation, 

Harry Nyquist, 6.1-NDRC-1481, BTL, Aug. 18, 
1944. Div. 6-625-Ml 

Chapter 15 

1. Tests 071 1-MC a7id 7-MC l7ite7'commu7iication Sys- 

te7ns of the USS Perch, Ralph C. Maninger and 
Wilbur T. Knudsen, Report P42/R759, NLL, Feb. 
17, 1944. Div. 6-623.41-Ml 

2. Suhmarme Inte7'7ial Commu7iicatio7i Systems 1-MC 

— 7-MC (Sum7nary of Studies, Reconunendations 
for Improvements and Descriptions of Modified 
System on USS Becuna), Don A. Proudfoot and 
Edwin E. Teal, Report D54/R992, NLL, June 28, 
1944. Div. 6-623.41-M3 

3. Speech a7id Hearmg, Harvey Fletcher. 

4. Factors Governing the Intelligibility of Speech, 
N. R. French, BTL, Sept. 21, 1942. 

5. Supe7'sonic U7iderwater Telephony, Victor V. Graf 
and Ray S. Alleman, NDRC 6.1-srll28-1560, Report 
P29/R715, NLL, Mar. 30, 1944. Div. 6-623.42-Ml 

6. Undei'water Telephony, J. Warren Horton, OSRD 
5183, NDRC 6.1-srll28-2211, Report D56/R1415, 
Service Project NS-248, NLL, May 15, 1945. 

Div. 6-623.42-M5 

7. An 8.2-Kc Single Side Ba7id U7iderwater Telephony 
System, Frank P. Herrnfeld, Report D56/R1395, 
Service Project NS-248, NLL, Feb. 28, 1945. 

Div. 6-623.42-M4 

8. The Effects of A7nplitude Distortion Upo7i the Intel- 
ligibility of Speech, OSRD 4217, Harvard Psycho- 
Acoustic Laboratory, Nov. 15, 1944. 

9. Sea Trials of U7ide)'water Telepho7iy Sy stern, Wil- 

liam F. Arndt, Report D56/PHR75, CUDWR, Jan. 
20, 1945. Div. 6-623.42-M3 

10. Comparison of 7-MC and M-MC Interrial Communi- 

cation Systems, Don A. Proudfoot and Edwin E. 
Teal, Report D54/R933, Service Project NS-212, 
NLL, May 20, 1944. Div. 6-623.41-M2 

11. Under'water Telephony by Means of Frequency 
Modidation, R. W. Beckwith, General Electric, 
Service Project NS-248, Oct. 6, 1944. 

Div. 6-623.42-M2 


CONTRACT NUMBERS, CONTRACTORS, AND SUBJECT OF CONTRACTS 


Contract Number 

Name and Address of Contractor 

Subject 

OEMsr-20 

OEMsr-1128 

The Trustees of Columbia University in the 

City of New York 

New York, New York 

The Trustees of Columbia University in the 

City of New York 

New York, New York 

Studies and experimental in- 
vestigations in connection 
with the development of 
1 listening and detecting sys- 

tems suitable for surface 
craft and for submarines. 

. Studies and experimental in- 
vestigations in connection 
with methods of furnishing 
harbor protection by means 
of cables and associated 
equipment. 

OEMsr-33 

RCA Manufacturing: Company, Inc., 

Camden, New Jersey 

Studies and experimental in- 
vestigations in connection 
with submarine and sub- 
surface warfare. 

OEMsr-692 

Western Electric Company, Inc., 

New York, New York 

Studies and experimental in- 
vestigations in connection 
with and for the develop- 
ment of equipment and 
methods pertaining to sub- 
marine warfare. 

OEMsr-695 

Western Electric Company, Inc., 

New York, New York 

Conduct studies and experi- 
mental investigations in 
connection with and for the 
development of equipment 
and methods involved in 
submarine and subsurface 
warfare. 

OEMsr-346 

Western Electric Company, Inc., 

New York, New York 

Studies and experimental in- 
vestigations in connection 
with the design and devel- 
opment of radio sonic 
buoys (capable of being 
dropped overboard from a 
ship) . 




203 


SERVICE PROJECT NUMBERS 


The projects listed below were transmitted to the Executive Sec- 
retary, NDRC, from the War or Navy Department through either 
the War Department Liaison Officer for NDRC or the Office of 
Research and Inventions (formerly the Coordinator of Research 
and Development), Navy Department. 


Se7'vice 

Project 

Number 


Subject 


AC-55 

NA-107 

NO-163 

NS-102 

NS-106 

NS-113 

NS-398 

NS-247 

NS-248 

NS-330 

NS-337 


Development of a directional radio-sonic buoy. 

Towed submarine listening gear for use with lighter- 
than-air craft. 

Cooperation with the Navy in harbor surveys and sur- 
veys of ambient underwater noise conditions in various 
areas. 

Development of subaqueous microphones for sono-radio 
buoys and cable connected hydrophones. 

Expendable sono-radio buoy. 

Listening apparatus for small patrol craft and sub- 
marines toroidal magnetostriction hydrophone. 

Consultant on contracts with Emerson Radio Phono- 
graph Corporation and Freed Corporation for manu- 
facture of expendable sono-radio buoys. 

Triangular ranging. 

Underwater voice communication system. 

Consulting services on production of radio transmitting 
equipments AN/CRT-4. 

WCA conversion equipments, consulting services on by 
Columbia University Division of War Research to 
BuShips (940) on its contracts NXsr-42164 (Task 9) 
and NXsr-65323 with RCA. 


INDEX 


The subject indexes of all STR volumes are combined in a master index printed in a separate volume. 
For access to the index volume consult the Army or Navy Agency listed on the reverse of the half-title page. 


Acoustic phase shift device, 9 
Aircraft detection by submarines, 0 
Aircraft listening equipment, 67- 
103 

radio sono buoys, 73-103 
towed hydrophones, 67-72 
Airplane noise, 19 
AM transmission, submarine com- 
munication, 189-191 
doppler shift, 190 
8-kc carrier, 190-191 
suppressed-carrier system, 190 
theoretical considerations, 189-190 
Ambient noise, 21, 54 
Amplidyne system for training hy- 
drophones, 115 

AN/ARR-3 ERSB receiver, 82-84 
AFC system, 82-84 
sensitivity, 83 

AN/ARR-16 DRSB receiver, 99- 
101 

Anchored radio sono buoy (ARSB), 
178-181 

comparison with cable-connected 
hydrophones, 172 
disadvantages, 169 
JM buoy, 178-181 
suggestions for improvement, 181 
training records, 181 
Anchored vessel screening (AVS), 
154-155 

AN/CRT-IA radio sono buoy 
see ERSB 

AN/CRT-4 radio sono buoy 
see DRSB 

Announcing system for submarines, 
183 

Antisubmarine applications of lis- 
tening, 4-5 

patrol craft systems, 4 
radio sono buoys, 4-5 
World War I ; 10 

ARSB (anchored radio sono buoy), 
178-181 

comparison with cable-connected 
hydrophones, 172 
disadvantages, 169 
JM buoy, 178-181 
suggestions for improvement, 181 
training records, 181 
ATF (automatic target follower) 
accuracy, 142 


crossed lobe principle, 16 
disadvantages, 118 
source of error, 143 
Attenuation of sound 

sonic frequencies, 20 ' 

supersonic frequencies, 11 
Automatic tracking, 692 sonar, 123, 
139 

AVS (anchored vessel screening), 
154-155 

Background noise, 21-22 
Bass-boost circuit, 39 
BDI (bearing deviation indicator), 
136 

Beacon systems, sonic, 7 
Beam width reduction, 43 
Bearing accuracy with crossed 
lobes, 13, 16 

Bearing deviation indicator, 136 
Bearing indicator systems, 50-60 
maximum indicators, 59-60 
null indicators, 51-59 
Bearing repeater, JT listening sys- 
tem, 115 

Bell Telephone Laboratories 
listening systems for small patrol 
craft, 23 
692 sonar, 122 

Bells, underwater (sonic beacon 
system), 7 

Binaural systems for bearing de- 
terminations, 9-10 
binaural compensator, 9 
electroacoustic application, 9 
harbor protection, 10 
line hydrophones, 9, 15 
performance, 9 

Blimp-towed hydrophone, 68-69 
acoustic performance, 69 
cable slip reel, 68 
construction, 68, 69 
towing performance, 69 
Block Island hydrophone system, 
171-172 

comparison with ARSB, 172 
components, 171-172 
hydrophones, 172 
maximum ranges, 172 
objectives, 171 

British torpedo detector, 154, 155 
Brush C-37 hydrophone, 172, 175 


Buoys, radio sono 
scd Radio sono buoys 

C-37 hydrophone, 172, 175 
C tubes (acoustic pickup units), 14 
Cable-connected hydrophones 
see Hydrophone system, cable- 
connected 

Cables, magnetic loop, 169 
Cape Henry hydrophone system, 
172-177 

attenuating networks, 174 
cable transmission characteris- 
tics, 175 

components, 174-175 
hydrophones, 175, 177-178 
installation, 170 
performance, 177 
switching circuit, 175 
terminating networks, 174 
Carbon button hydrophones, 7 
Carbowax plug 
DRSB, 93 
ERSB, 78 

Cathode-ray phase indicator, 57-59 
Cavitation noise, 17-19 
submarines, 18 
surface vessels, 17-18 
CDI-50123 amplifier, 175 
Close-talking microphones, 183 
Columbia University, Division of 
War Research, 23 

Communication systems for subma- 
rines, 182-193 

intercommunication, 187-193 
intracommunication, 182-186 
Compass repeater, MTB equipment, 
123 

Compass-capacitor in DRSB, 87-90 
construction, 95 
double-pivot type, 88 
slave-master principle, 90 
Continuous search indicator, 59 
Continuous search listening, 692 
sonar, 123, 137, 139 
Converter-amplifier, JT listening 
gear, 112-113 

Convoy protection buoy, 73 
Crossed lobes for bearing accuracy 
description of method, 13 
use in automatic target follower, 
16 


205 


206 


INDEX 


Crystal projector WFA, three-sec- 
tion, 122 

DDI (direct deviation indicator), 
141-142 

Delobed hydrophones, 105, 120 
Destroyer noise, 157-158 
Directional hydrophones, 62, 74 
Directional radio sono buoy 
see DRSB 

Directivity index, 29, 43 
Directivity patterns, transducer 
effect of dome, 38 
electrically steered sonic array, 
28-31 

electrically steered supersonic ar- 
ray, 33-34 

mechanically steered sonic ar- 
ray, 36 

mechanically steered supersonic 
array, 38-39 

692 sonar projector, 128-130 
through-the-hull sonic listening 
system, 39-41 
Domes, 37-38 

Doppler shift in modulated signals, 
190, 193 

DRSB (directional radio sono 
buoy), 87-103 
carrier frequency, 101 
effective operating life, 101 
geographical bearing, 5 
operation, 101 
performance tests, 101-103 
receiver, 99-101 
targets, 101-103 
DRSB design, 91-101 
bottom section, 95-98 
buoy housing, 91-93 
cap section, 98-99 
carbowax plug, 93 
color-coded buoys, 93 
compass-capacitor, 14, 87-90, 95 
dye pack, 99 

hydrophone, 74, 87-88, 96-97 
motor assembly, 97-98 
orientation system, 95 
parachute, 98 
security provision, 93 
transmitter section, 93-95 
DRSB developmental models, 87-91 
compass-capacitor, 87 
gravity-type motor, 88 
hydrophone, 87-88 
Mark I buoy, 88-89 
Mark II buoy, 89-90 
Mark III buoy, 90 
Mark IV buoy, 90 


methods of rotating hydrophone, 
88 

orientation reference system, 87- 
88 

Echo ranging 

antisubmarine warfare, 1 
disadvantages, 1 
submarine warfare, 5 
Electrically steered listening sys- 
tems, 23-34 
sonic listening, 23-32 
supersonic, 32-34 

Electron ray level indicator, 59, 63 
Electron ray tube, JP-1 equipment, 
107 

E-P torpedo detector (Electro-Pro- 
tective Corporation) 
hydrophones, 154 
method of functioning, 160 
suggested improvements, 163 
ERSB (expendable radio sono 
buoy), 13, 74-86 
launching methods, 85 
operation, 85-86 
receiver, 82-84 
requirements, 73 
search procedure, 86 
slicks and markers, 86 
suggested improvements, 86 
ERSB design, 78-81 
antenna, 81 
battery supply, 80 
carbowax plug, 78 
dyes for identification, 86 
housing, 78 
hydrophones, 79 
parachute, 81 
transmitter, 79-80 

Fessenden oscillator, 7 
Float light, Mark V, 86 
Fluorescein dye for radio sono 
buoys, 86 

FM transmission, submarine com- 
munication, 187-189 
failure of FM system, 189 
modified system, 188 
power-line carrier system, 187 
Frequency response 

electrically steered sonic array, 
27-28 

electrically steered supersonic 
listening system, 33 
hydrophones, 43 

mechanically steered sonic array, 
36 

mechanically steered supersonic 
array, 38 



692 sonar projector, 127 
through-the-hull sonic listening 
system, 39 

Frequency spectrum of listening 
gear, 3, 11-12 , 

Graphic recorder, 172 

Harbor protection systems, 169-181 
anchored radio sono buoy, 178- 
181 

binaural listening gear, 10 
cable-connected hydrophones, 
170-178 

HW towed hydrophone, 69-71 
acoustic performance, 70 
binaural tests, 70 
construction, 69 
towing performance, 71 

Hydrophone system, cable-con- 
nected, 170-178 

Block Island installation, 171-172 
cable loading, 178 
Cape Henry installation, 172-177 
comparison with anchored radio 
sono buoy, 172 
.design problems, 170-171 
equalization of signal levels, 178 
hydrophone matching networks, 
178 

recommendations, 177-178 
recorders, 172 
relay switching circuit, 178 
splicing procedure, 178 
timing circuit, 178 

Hydrophones, characteristics 
angle of incidence, 23 
bearing accuracy, 4 
frequency response, 43 
noise threshold, 24 
phase-cancellation principle, 24- 
27 

recommendations, 48 
response formulas, 50-51 

Hydrophones, general types 
binaural system, 9, 15 
blimp-towed, 68-69 
cable-connected, 169-178 
carbon button, 7 
delobed, 105, 120 
directional, 62, 74 
line hydrophones, 6, 14-15, 52-53, 
67-69 

magnetostriction, 62-64, 79, 87- 
88 

multispot arrays, 9-12 
permanent magnet, 35-36 
Rochelle salt crystal, 32-33 
symmetrically tapered, 51-53 


INDEX 


207 


toroidally wound, 64, 75-79 
towed, 67-72 

Hydrophones, specific types 
Brush C-37; 172, 175 
H\V towed hydrophone, 69-71 
JP, 41, 107, 146-147 
JT, 108-109 
magnetophone, 8 
NL-124; 109, 148, 149, 177-178 
NL-130; 118 

Image effect of sound, 20 
Indicators, bearing, 50-60 
maximum indicators, 59-60 
null indicators, 51-59 
Indicators, visual 
automatic target followers, 16, 
118, 142-143 

cathode-ray phase indicator, 57- 
59 

torpedo detectors, 15-16 

JK supersonic gear, 106 
JM buoy, 178-181 

experimental model, 180-181 
production buoy, 181 
JP overside systems, 61-66 
design principles, 61 
overside equipment, 62-64 
preliminary work, 62 
suggested design improvements, 
66 

through-the-hull equipment, 64- 
66 

JP-1 listening equipment, 105-108 
amplifier, 107 

circuit for magnetizing hydro- 
phone, 107 

hydrophone and baffle, 41, 105, 
107, 146-147 
magic-eye tube, 107 
mounting and training mecha- 
nism, 107 

noise monitoring, 108 
performance, 107-108 
self-noise, 21-22 
visual detection, 107 
JP-2 hydrophone, 105 
JP-3 hydrophone, 105 
JT listening system, 108-121 
amplidyne system, 115 
baffle, 109-110 

bearing repeater system, 115 
frequency range, 109 
hydrophone, 109 
performance, 121 
pilot model, 118-119 
production model, 119-121 
RLI and listening amplifier, 109, 
112 


sonar talkback system, 115-117 
sound absorbing coupler, 110-112 
supersonic converter, 112-113, 
119 

test and calibration facilities, 
118 

training mechanism, 114-115 

Line hydrophones, 6, 14-15 
anchored buoys, 15 
applications, 14-15 
bearing accuracy, 48 
binaural effect, 15 
magnetostriction, 67-69 
through-the-hull mountings, 14 
uniform-line, 52-53 
Listening, general discussion, 1-6, 
13-22 

advantages, 1-3 

antisubmarine applications, 4-5 
bearing accuracy, 4 
binaural effect, 15 
continuity of information, 2 
equipment design considerations, 
3-4 

factors affecting performance, 
3-4 

line hydrophones, 14-15 
oceanographic conditions, 3 
prosubmarine program, 5-6 
radio sono buoys, 13-14 
range, 2, 3, 45-46 
simplicity of equipment, 3 
sound attenuation, 2-3, 11 
tactical security, 1-2 
target signal characteristics, 17- 
22 

visual indicators and recorders, 
15-16 

wide-band coverage, 3 
Listening, World War I; 7-10 
acoustic systems, 8 
antisubmarine gear, 10 
bearing determinations, 9-10 
Fessenden oscillator, 7 
frequency coverage, 8 
harbor protection, 10 
magnetophone, 8 
multispot arrays, 10 
towed systems, 8 
underwater bells, 7 
Listening, World War II; 11-16 
applications, 13-16 
crossed lobes, 13 
electroacoustic transducer status, 
12 

methods of improving detection, 
13 

multispot arrays, 11-12 


optimum frequency characteris- 
tics, 11-12 

supersonic listening, 11 
Listening systems, characteristics 
beam width and side lobe reduc- 
tion, 43 

comparative ratings, 45 
directivity index, 43 
directivity patterns, 28-41 
frequency response, 27-43 
frequency spectrum, 3, 11-12 
noise, 30-34, 38-41, 44-45 
rear response, 45 

Listening systems, experimental, 
23-34 

electrically steered systems, 23- 
34 

mechanically steered systems, 
34-38 

through-the-hull sonic system, 
39-41 

Listening systems, sea tests, 45-48 
bearing accuracy and listening 
range, 45-46 
ease of operation, 47 
target interference, 46 
Listening systems, types 
electrically steered, 23-34 
JP overside systems, 61-66 
JP-1 equipment, 106-108 
JT, 108-121 

mechanically steered, 34-38 
radio sono buoy, 4-5, 13-14, 73- 
103 

recommendations, 48-49 
692 sonar, 122-139 
through-the-hull sonic listening, 
39-41, 64-66 

towed listening gear, 8, 67-72 
triangulation listening ranging, 
140-153 

Lloyd mirror effect, 20 
Lobe reduction, 43 
Lobes, crossed 

description of method, 13 
use in automatic target follower, 
16 

Loop cables, magnetic, 169 

MAD (magnetic airborne detec- 
tor), 73 

Magic-eye tube, 63, 66, 107 
Magnetic loop cables, 169 
Magnetic-tape loop recorder, 172 
Magnetophone, 8 

Magnetostriction hydrophones, 62- 
• 64 

for directional radio sono buoy, 
87-88 


20» 


INDEX 


for expendable radio sono buoy, 
79 

for overside listening equipment, 
62-64 

for through-the-hull equipment, 
64 

line hydrophines, 67-69 
Maintenance of true bearing 
(MTB), 692 sonar, 123, 138 
Mare Island, JP training mecha- 
nism, 114 

Mark I-IV radio sono buoys, 75-77, 
88-90 

Mark V float light, 86 
Markers for ERSB, 86 
MATD (mine and torpedo detec- 
tion), 163 

Maximum indicators for bearing 
determinations, 59-60 
continuous search, 59 
electron ray level, 59 
Mechanically steered listening sys- 
tems, 34-38 

sonic listening, 34-37, 39-41 
supersonic listening, 37-39 
Merchant vessel protection against 
torpedoes, 154-156 
detection equipment, 158-163 
suggested improvements, 154-155 
Meter-actuating circuits 
PAL, 30, 33, 51-55 
VBI, 55-57 

Microphones, submarine communi- 
cation system, 183-184 
close-talking microphones, 183 
recommendations, 184 
speaker microphones, 184 
Mine and torpedo detection, 163 
MTB (maintenance of true bear- 
ing), 692 sonar, 123, 138 
Multispot hydrophone arrays, 9-12 
MVP torpedo detector (merchant 
vessel protection), 158-163 
circuit analysis, 160-162 
continuously rotated projector, 
158 

evaluation, 163 
field performance, 162 
tests and observations,^ 159, 162- 
163 

warning time, 162 

New London Laboratory 

hydrophones, 109, 118, 148, 177 
submarine communications sys- 
tems, 182-193 

New Portsmouth, JP training 
mechanism, 114 
NL-124 hydrophone, 109 
advantages, 177-178 


sensitivity, 148 

use without lobe reduction, 148- 
149 

NL-130 hydrophone, 118 

NLM (noise level monitor), 108 

Noise 

see also Self-noise 
airplanes, 19 
ambient, 21, 54 
background noise, 21-22 
cavitation noise, 17-19 
destroyer noise, 157-158 
submarine noise, 18-19 
surface vessels, 17-18 
target noise, 17-22 
torpedo noise, 19, 155-156 
wind noise, 183 
Noise level monitor, 108 
Noise measurements, 155-158 
destroyers, 157-158 
merchant ships, 156-157 
torpedoes, 155 

Noise threshold of hydrophones, 24 
Null indicators for bearing deter- 
mination, 51-59 

cathode-ray phase indicator, 57- 
59 

effectiveness, 119 
for JT listening gear, 109, 112 
for triangulation listening rang- 
ing, 141, 146-147, 150-151 
PAL indication, 51-55 
source of error, 143 
submarine installation, 150-151 
VBI indication, 55-57 

Oceanography, effect on listening 
performance, 3, 20-21 
Old Portsmouth, JP training mech- 
anism, 114 

1-MC submarine communication 
system, 183 

, Oscillator, Fessenden, 7 

Overside listening equipment, 62-64 
amplifier and power supply, 63 
hydrophone shaft and training 
mechanism, 62-63 
performance, 64 

PAL (phase-actuated locator), 51- 
55 

ambient noise, 54 
AVC, 51, 54 

band of frequencies, 53-54 
bearing accuracy, 46 
inaccuracy in phase-shift net- 
work, 55 

interfering signal, 54-55, 128 
symmetrically tapered hydro- 
phones, 51-53 


uniform-line hydrophones, 52-53 
use with electrically steered ar- 
rays, 30, 33 

Patrol craft systems, application of 
listening gear, 4 

Phase indicator, cathode-ray, 57-59 
Phase shift device, acoustic, 9 
Phase-actuated locator 
see PAL 

Phase-cancellation principle, hy- 
drophone array, 24-27 
Potentiometer, scale expander, 99 
Power-line carrier system, subma- 
rine communication, 187 
PPI (plan position indicator), 123 
Projectors 

continuously rotated, 158 
692 sonar, 125-134 
submarine intercommunication 
system, 191 

with streamline dome, 167-168 
Prosubmarine program, 5-6 
detection of aircraft, 6 
detection of torpedoes, 163-168 
sonar installation, 5 
use of echo ranging, 5 
use of listening, 5-6 

QBF dome, 37-38 

Radio Corporation of America 
convoy protection buoy, 73 
JT sonar equipment, 119 
Radio sono buoys, 73-103 
aircraft detection, 6, 13-14 
antisubmarine applications, 4-5 
ARSB, 169, 172, 178-181 
convoy protection buoy, 73 
DRSB, 14, 87-103 
ERSB, 13, 74-86 
experimental work, 75-77, 87-90 
Range of listening systems 
comparison of systems, 45-46 
maximum range, 2 
signal-to-noise ratio, 3 
Range recorder, Sangamo, 166 
Ranging by triangulation listening 
see Triangulation listening rang- 
ing 

Rear response of listening systems, 
45 

Recognition frequency of sound, 21 
Recommendations for future re- 
search 
ARSB, 181 

cable-connected hydrophones, 
177-178 

E-P torpedo detector, 163 
ERSB, 86 

listening systems, 48-49 



INDEX 


209 


submarine internal communica- 
tions systems, 186 
throug:h-the-hull listening: equip- 
ment, 66 

triangfulation listening: rang:ing:, 
153 

Recorders, visual, 15-16 

automatic targ:et followers, 16, 
118, 142, 143 
torpedo detectors, 15-16 
Reflection of sound, 20-21 
Rhodamine dye for ERSB, 86 
RLI (rig:ht-left indicators) 
see Null indicators for bearing: 
determination 

Rochelle salt crystal hydrophone, 
32-33 

Sang:amo sound rang:e recorder, 
166 

Scale expander potentiometer, 99 
Scattering: of sound, 20 
Security of listening: methods, 1-2 
Self-noise 

electrically steered arrays, 30-33 
JP-1 grear, 21, 107-108 
mechanically steered array, 36- 
37, 39 

ships, 156-157 
692 sonar, 123 
sources, 30-31 

throug:h-the-hull sonic listening: 
system, 41, 44-45 

Sensitivity differential of instru- 
ments, 15 

7-MC submarine communication 
system 

comparison of articulation per- 
centages, 185 
feedback, 183 
modified system, 184-185 
recommendations, 183 
wind noise, 183 
Shadow zone, ocean, 20 
Ship self-noise measurements, 156- 
158 

destroyer noise in streamlined 
domes, 157-158 
merchant ships, 156-157 
Shock waves, 19 
692 sonar, 122-139 
bearing accuracy, 123 
components, 125 

performance and conclusions, 
138-139 

requirements, 123 
self-noise, 123 

692 sonar, listening equipment, 
134-138 


automatic tracking, 123, 139 
BDI, 136 

continuous search, 123, 137, 139 
control panel, 137 
detector panel, 137 
hand training, 137 
indicator panel, 136-137 
modulator panel, 137 
MTB panel, 123, 138 
operation, 134-136 
power supply panel, 138 
sound level panel, 134-136 ^ 

692 sonar, projector, 125-134 
characteristics, 127-130 
crystal arrays, 127 
directivity patterns, 128-130 
frequency response, 127 
projector training system, 130- 
134 

specifications, 125-127 
speed control system, 133 
Sonar talkback system, 115-117 
see also 7-MC submarine com- 
munication system 
Sonic beacon systems, 7 
Sonic listening 

see Listening, general discussion 
Sonic listening systems, electrically 
steered, 23-34 
directivity patterns, 28-31 
disadvantages, 34 
hydrophones and circuits, 23-27 
measured characteristics, 27-32 
noise, 30-32 

Sonic listening systems, mechani- 
cally steered, 34-37 
directivity patterns, 36 
frequency response, 36 
hydrophone and circuits, 35-36 
measured characteristics, 36-37 
noise, 36-37 

Sonic listening systems, through- 
the-hull mounting 
see ThVough-the-hull listening 
system 
Sono buoys 

see Radio sono buoys 
Sound attenuation 
sonic frequencies, 20 
supersonic frequencies, 11 
Sound reflection, 20-21 
Sound scattering, 20 
Speaker microphones, 184 
Sperry Gyroscope Company, 144 
Spreading of sound, 20 
Submarine bells, 7 
Submarine defense, 5-6 
detection of aircraft, 6 
detection of torpedoes, 163-168 
sonar installation, 5 


use of echo ranging, 5 
use of listening, 5-6 
Submarine intercommunication 
system, 187-193 

amplitude-modulated transmis- 
sion, 189-191 

band-pass filter circuit, 191 
directionality of projectors, 191 
effect of supersonic waves, 187 
frequency modulated transmis- 
sion, 187-189 

power-line carrier system, 187 
prototype equipment, 191-193 
requirements, 193 
transmitting circuit, 191 
Submarine intracommunication 
systems, 182-186 
modified system, 184-185 
1-MC system, 183 
performance of microphones, 
183-184 

recommendations, 186 
7-MC system, 183-185 
Submarine listening systems, 104- 
153 

comparison of sonic and super- 
sonic gear, 106 

design considerations, 105-106 
early models, 106 
factors controlling range, 104 
JP-1 equipment, 106-108 
JT system, 108-121 
692 sonar, 122-139 
triangulation listening ranging, 
140-153 

Submarine noise, 18-19 
Supersonic carrier telephony 
see Submarine intercommunica- 
tion system 

Supersonic converter, JT listening 
gear, 112-113, 119 
Supersonic listening systems, elec- 
trically steered, 32-34 
directivity patterns, 33-34 
frequency response, 33 
hydrophones and circuits, 32-33 
measured characteristics, 33-34 
noise, 34 

Supersonic listening systems, me- 
chanically steered, 37-39 
directivity patterns, 38-39 
hydrophone, dome and circuit, 
37-38 

measured characteristics, 38-39 
noise, 39 

Suppressed-carrier system, subma- 
rine communication, 190 
Surface craft listening equipment, 
61-66 

design principles, 61 




210 


INDEX 


overside equipment, 62-64 
preliminary work, 62 
suggested design improvements, 
66 

through-the-hull equipment, 64- 
66 

Surface craft noise, 17-18 

Tactical security of listening 
methods, 1-2 

Talkback system, sonar, 115-117 
Target follower, automatic 
accuracy, 142 
crossed lobe principle, 16 
disadvantages, 118 
source of error, 143 
Target noise, 17-22 
airplane noise, 19 
background noise, 21 
cavitation, 17 
explosion waves, 19 
machinery, 17 
submarine targets, 18-19 
surface vessel targets, 17-18 
torpedo noise, 19, 155-156 
transmission losses, 20-21 
versus background noise, 21-22 
Target tracking, 692 sonar, 123, 
139 

TDM (torpedo detection modifica- 
tion), 163-168 
development, 165-166 
projector with streamline dome, 
167-168 

requirements, 168 
rotation of projector, 165-166 
tests, 167-168 
visual indicator, 166 
Telephony systems, underwater 
see Submarine intercommunica- 
tion system; Submarine intra- 
communication systems 
Threshold of noise, hydrophones, 
24 

Through-the-hull listening system, 
64-66 

directivity patterns, 39-41 
hoisting -and -training mecha- 
nism, 65-66 

hydrophone and baffle, 64 
modified system, 158-159 


performance, 66 
recommendations, 66 
response characteristics, 39 
self-noise and internal noise, 41, 
44-45 
TLR 

see Triangulation listening rang- 
ing 

Topside listening equipment 

see Submarine listening systems 
Toroidally wound hydrophone, 64, 
79 

Torpedo detection modification 
see TDM 

Torpedo detection systems, 6, 154- 
168 

anchored vessel screening, 154- 

155 

British, 154, 155 
E-P torpedo detector, 154, 159- 
160, 163 

for merchant vessels, 154-156, 
158-163 

for submarines, 163-168 
ship self-noise measurements, 
156-157 

torpedo noise measurements, 155- 

156 

visual indicators, 15-16, 166 
Torpedo noise, 19, 155-156 
Towed listening gear, 8, 67-72 
acoustic performance, 69, 70 
blimp-towed, 68-69 
cable drag, 67 
construction, 68, 69 
general curve for towing cables, 
71-72 

HW towed hydrophone, 69-71 
mechanical considerations, 67 
status of development, 72 
towing performance, 69, 71 
Transducers, World War II; 12 
Transmission loss in water, acous- 
tic, 20-21 

Triangulation listening ranging, 
140-153 

ATF mechanism, 16, 142 
bearing accuracy factors, 142- 
143 

bearing deviation indication, 141 


direct deviation indicator, 141- 
142 

effect of interfering targets, 153 
hydrophones, 144-149 
preproduction units, 152-153 
ranging accuracy, 140 
recommendations for future de- 
velopment, 153 

RLI and listening equipment, 
141, 146-147, 150-151 
servoamplifier, 144-145, 151 
submarine tests, 143-144, 149-152 
surface ship tests, 141-143 
tactical requirements, 140-141 
triangle solver and recorder, 144- 
146, 151 

USS S48 installation, 144-148 
TTH listening system 

see Through-the-hull listening 
system 

Underwater bells, 7 
Underwater telephony systems 
see Submarine intercommunica- 
tion system; Submarine intra- 
communication systems 

VBI (vector bearing indicator), 
55-57 

Visual indicators, 15-16 

automatic target followers, 16 
differential sensitivity, 15 
for torpedo detectors, 15-16, 166 
Voice-modulated underwater te- 
lephony 

see Submarine intercommunica- 
tion system; Submarine intra- 
communication systems 
Volume level indicator, 59 

WCA-2 torpedo detector 
see TDM 

WFA three-section crystal pro- 
jector, 122 

XJAA listening -ranging equip- 
ment, 152 

XN-1 radio sono buoy 
see DRSB 


4 



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